For business use percentage, I'm using the "tax court" method, where I provide the number of days of rental and the number of days of personal use (not counting the days I was working on the place or preparing for renters in either figure). Last year it was 10 rental days and 11 personal days, hence 10/(10+11) = 4762.%. This year it was 72 rental days and 14 personal days, hence 72% business use. I believe this calculation is correct. Vacant periods don't count as business or personal.
I think you're right about what Turbotax is doing wrong. But I don't see why it's complex to fix. The calculation is much simpler if it doesn't attempt to make up for taking too little depreciation in previous years because business use percentage changed. It's simply the basis times the percent of business use times the appropriate number from Table A-6 on p, 71 of IRS Publication 946.
