50% of a 77% turnout is 35%. The number of votes cast for the SNP in the election was about the same as those that voted Yes in the independence referendum, and that was *BEFORE* the collapse in the Oil price.
Personally if I was David Cameron I would ram fiscal independence down Sturgeons neck. I would have it coming into full force in April 2017, which is slower than Salmond wanted, because he was going for *FULL* independence in May 2016. When faced with the reality of devastating cuts to public spending and/or massive tax rises that fiscal independence would bring the SNP might just not do as well next May. Of course if the SNP did not win next May you could back peddle on it :-)
Business taxes is the difficult one, we know the SNP plan is to have a lower rate than England so they can leach taxes from England by getting multinational corporations to relocate their headquarters to Scotland. Two solutions to that in my view, the simpler one is just match anything the SNP do and play a game of chicken that Scotland will loose. The more complex one is say force companies with a profit over say £1m to make up the difference lower Scottish taxes based on percentage of turnover generated in the rest of the UK removing the benefit of a multinational relocating to Scotland to avoid tax and sinking the SNP's taxes plans before they get out of harbour.
The other thing I would do is ram another independence referendum down the SNP's throat. When the loose for a second time they are going to have a hard time calling a third for decades, by which time there should be no oil left and their economic plans will be in complete tatters.