Aaron, I wished I had an answer for ya, but frankly I never really thought about it?
I think that most of my bifaces are thinner( ratio wise) that my preforms, and after I get it to preform size, then I know that I am getting closed to done,so the point usually winds up close to the same ration as my preform. I need to watch this next time I build some and see how it holds up.
Good looking Dacite. I never get to hit that stuff much. It does make some awesome looking Paleo stuff.