Here's my point of view of a very similar process undertaken here in Quebec. Our equivalent to the SNP was first elected in 1976. Its creators had debated previously if the road to independence should be done by passing a vote declaring it in the National Assembly or by first setting up a popular referendum on the question. The leaders opted for this 2nd option and launched the referendum 4 years into power. The result was 40%/60% against. The government was nevertheless liked by the people and re-elected soon after. The question of independence, the basic reason for the party, was partially transferred to demands for greater autonomy and partially awkwardly stored on a shelf. The party then lost the 2 next elections and got back in power in 1994. The political mood had changed and its new leader set up a fresh referendum in 95. This time the vote was 50%/50% (with a revealing 60% support in favor when taking into account only "root Quebeckers", meaning without votes from immigrant communities). Here again, the people liked the party and re-elected it. The party concentrated on "good government", and answered partisans wishing a 3rd referendum that to make sure we got over the 50% this time we had to wait until "winning conditions" were there (broadly meaning a comfortably leading support in the polls). The risks of setting up a 3rd failing referendum were judge too great for the cause. After ten years in power, the popular mood was for change and the party lost the elections. The party has since been split between those who want a 3rd referendum as soon as possible, those who want a 3rd referendum when the polls look promising enough to win, and those who argue the party should consider declaring independence by a vote in the Assembly once elected to a majority government (these argue that the referendum process has demoralized supporters (2 losses in a row, the dirty tricks used by the opponents...) and proven itself defenseless against the counter-measures taken by the Canadian government to discredit its legal value. These 3 split point of views have spurred the creation of 2 small new independence parties. The main party is currently in the process of trying to address their concerns to reunite people behind the cause.

Now that this is said, my own personal opinion on the matter and what the SNP should keep in mind to succeed... It's not a bad thing to be a pro-independence party temporarily governing a province (or a sub-national territory, a "Kingdom" in Scotland's case I guess). You get to set a lot of the groundwork for independence without getting there yet, plus you find your opponents forthcoming in making concessions, as they believe this will satisfy you and settle the matter. Most importantly, as you run the day-to-day business of the government, you gain credibility in the voter's heart, they see that you're a serious, responsible government, a government they can trust enough to give a country to. It removes any unfounded perception of radicalism some voters might interpret in a pro-independence party. The party becomes trusted on a large-scale level, essential for what follows. Now this said, there comes a time when all possible concessions to be obtained have been obtained. There also comes a time when the confidence and credibility a government in power has built starts to fade, elections have cycles, parties alternate as popular mood changes. So when to make a move for independence and how? Unless a referendum is statistically certain to result in a positive outcome, I would argue that it should be avoided. A loss can only demoralize supporters and make them feel helpless enough to make some think the matter is closed, not to mention putting your opponents in a position of strength and self-justification. True another referendum can take place anytime but odds are it would have to wait several years, a decade most probably (it took 15 years here). Apart from true hard-core supporters, most people, even soft nationalist won't see the point in setting up another consultation a year later (unless a major event comes in to change perceptions). Personally, I feel that once a pro-independence party has made its name as a credible governing party, and obtained all concessions possible from its opponents (locally and at the central government), it should adopt the intent of declaration of independence through a vote in parliament, and ideally after a break from power. A freshly elected party has momentum. It can state its intentions clearly in a regular election campaign, people who want this party to pass a vote declaring independence should vote for it, others should be made well aware not to. Since the party will have shown it could govern and govern well before (and obtain concessions), its voters' base will be large and mainstream, whatever the perception some might have of its apparently radical program. Possible opposition to a non-referendum-based road to independence will be minimalized. This said, it is imperative that the party following this intent be elected as a majority government, it must have obtained both a majority of seats and a majority of votes (sometimes these don't follow). This is essential not only to make sure the vote for independence passes in parliament but to make sure the vote has the vested democratic authority of the people. Opponents will try to argue in vain that only a referendum on the question is democratic. This is nonsense. If the party respects the double majority criteria and makes clear to the voters what its intent is, a regular general election has all the democratic weight necessary to legitimize an open parliamentary vote declaring sovereignty. What happens if the party obtains a minority government? I would answer that if the party already has governed in the past with credibility, they should either forfeit government to the runner-up or govern as the best government possible would do until the next election (without compromises, the fall of its minority government being at its advantage). One way or the way it is imperative that it avoids entrenching itself as regular provincial (regional) governing party. Like I explained before, this can lead in the long run to a split between pro-independence supporters and even make the people see the cause as a tired cause that never materializes. Opponents who say the double majority electoral road to independence doesn't take into account the ordinary citizens' wish are wrong. Did someone ask the Scottish people in 1707 if they wanted to join the UK? Apparently not, it was apparently voted in parliament. So you could vote in but could not vote out? I'm open to discuss more on the matter to those who wish so. This stuff is none of my business actually, I'm a Quebecker of mixed French (settled in 1640) and Scottish (settled in 1801) ancestry. I support Quebec independence (voted yes in 95) and I'm naturally inclined to take interest into the Scots' cause. There are undeniable parallels in the process.