Never doing this before(disclaimer), I would put the existing hole as your "baseline " so to speak. Placed in vice, oriented straight up.Get the orientation figgered out(15* )mentioned earlier and make sure to not "open up" existing hole too much.
The larger the disc the more gradual the new "groove".
Disclaimer inserted lol
Absolutely agreed with deeper in the center and taper out to nothing on the end but again there's also the roundness of the shaft to consider? Correct?
