1. listed above is for layout purposes only on a symmetric ball to make it easier to do while using the dual angle layout method. It's actual reference to the psa isn't a factor but more so on the cg location and the need for a possible weight hole later.
The point written above by Aloarjr810 about the ending location of the psa on a symmetric ball is your most accurate way of guesstimating. The end results is the psa will always be in the direction of the thumb hole on a symmetric core ball. Even when adding a large weight hole out towards the vertical axis line the psa will only move slightly.
Most cases the high rg axis and the low rg axis move in the area of 3/8" or less from undrilled to drilled. So on any symmetric core ball already drilled, take a prosect and draw a line 6 3/4" from the pin through the thumb and mark the area. You will be close enough.
A great way to see this in action and better learn for yourself is by down loading a free trial version of the blueprint software.
http://blueprintbowling.com/
