You want adequate clearance so the vanes do not contact your face or nose at full draw. Everyone’s anchor and facial/nose shape are slightly different. That said the back of my vanes are 1-5/8” in front of the nock groove on my arrows. Any closer and the vanes hit my nose.
I’ve never actually measured it. I have them as close to the nock as I can without causing any contact with my face. I put a mark on my Bitz and always line me up with that but would guess 1.25-1.5”