Yep I agree the tightest to the ankle bowlers I know are near 6 boards from their slide foot. (Gives them 1.5 boards of leeway from having ball strike ankle).
More typically 7 to 8 boards.
I'm a moderately big fellow. I would not want to put the ball down 15 boards from my center point. (Base of my spine) I believe this would be about 10 boards from my ankle and would leave me quite off balance.
I like the Dick Ritger method, contained on tape 2. That discusses Your starting point, your drift and then moves your setdown point 7 boards from that point.
ie I drift two boards towards the inside. For me to throw the ball straight down 10 I start 15 drift to 17 and throw straight down 10!
REgards,
Luckylefty