Yup, I get stuck every time I'm about to take what feels like a "high risk" step. I totally understand your hesitation.
My advice is to check you calculations, check them a second time, and then check them a third. Then ask yourself:
1. What's the worst that can happen if you're wrong?
2. If "the worst" happens, how might you proceed?
If your plan seems logical and you can imagine a Plan B (even if it's not as good as Plan A) JUST DO IT!
In fact, the worst that can happen is that you won't produce any quilt at all. That would be very sad!