You're going the wrong way with the backspace math. You're thinking correctly, though... If a 15x7 with 4-1/4" bs gets you centered, then a 15x8 will want 4-3/4". when you measure a rim, remember we're measuring to the tire bead, not the outside edge of the rim.

Just checked out your post re: your motor. I have a lot of experience with my 2-bbl / 270 horse 383. If you have any questions, don't hesitate to PM me.