jdupre,
You are actually talking about something a little different than what oldogrib is talking about. You are talking more about a point of aim than a gap.
And you are right....if you put your arrow tip ON something at about 15 yards, as you walk back you can pretty much keep your point ON the same spot and still hit close to your mark.
This is because, as you mentioned, the overall sight picture gets smaller as you move back, thus making your gap get smaller as you back up even though your POINT OF AIM remains the SAME.
Obviously it doesn't work all the way back to your point on, but it's a great tool to know for hunting situations (ex: point on deer hoof as long as you are between 15 and 30 yards for example)