I'm making my first self bow. This one will have a shelf, but my question is, on the average how much above center is the shelf?
The average hand is 4 inches wide. That's a biblical "Handbreath", Pastor. I center my handle but some like to have their top limb 1 1/2" longer and compensate the handle to accomadate that. Either way, take your handwidth of 4", then add a bit of fudge room, say, 1/2" to allow gloves and hand movement. 4 & 1/2" of handle, directly above that will be the shelf of your choice should work well.
If you can get a copy of "Hunting The Osage Bow" it will teach you volumes.
I've been measuring that specification on self and glass laminated longbows and recurves bows for the past 40 years, probably more than 100 bows. The measurements ranged from 1 1/8 to 2 1/4 inches above the center of the bow.
Here's some interesting reading for you as well.
tillering (http://www.bowyersedge.com/organic.html)
About 1.25 in above center. But this may vary according to the tiller but not usually. Jawge