I tried checking the Yes on 10a and sure enough it corrects the percentage. And that seems to only effect lines 22 & 23 but those don't flow through because line 25 (Standard mileage) is checked, and line 28e which only has a tiny effect on the end result because the new car was registered so late in the year that was a small number.
My wife's RE Office is in our house, so she tracks all her RE miles showing houses, etc. and there are no commuting miles. By definition anything not business miles is personal miles, but they aren't specifically tracked in her log book. She does also track medical and volunteer miles for use in other areas of the tax forms, but those are personal miles too.
So I can't legitimately check Yes on line 10a and guess I have to accept the end result, but I argue that the instructions for calculation of line 9 are clear and shouldn't be effected by things lower in the worksheet.
