My question is how much back set is to much starting a 68" form and would like to get it  right  the first time. Thanks

What style bow? I think that would make a difference.

68" flat back glass lam.

3/4 to 1 should be fine. I use 1 for 68 ntn. Out of the form and finished  tillering gives slightly less.

I have gone with three inches and had no problems.  Backset on longbows is a very personal preference feature.  It will increase the pre-load and allow a little more energy storage.  If this is a first,  I'd suggest about an inch give or take.