I have also found that I need to tweak my 'final design' a few times before I'm happy with it. My method is to use a hunk of flat cereal box to make a pattern - it's just flexible enough to imitate a piece of windsheld material. It might make 2-3 tries before I get the windshield the way that I want. But the changes literally only take 3-4 minutes to do once you've made your first one.

Then I will mark the fuselage so that the pattern goes right back down in the same spot, every time. Then the pattern is cut out of the real material.

I had a few problems myself until I decided to use cereal boxes for patterns...

Just my $.02

Bob