IMO this whole thread is full of holes. In the first place if the driller does not have the skill, ability and correct tools to drill a symmetrical to shift cg to correct position and measure PSA before x hole on gradient line and after x hole and placement P1, P2, P3, P4 or no x hole this whole supposition about how the ball should be drilled for said bowler is much ado about nothing. Driller needs a DeTerminator to determine PSA. Everything else is conjecture for the most part.
So, I will agree with those that said whether the cg is stacked or mid grip means little if the driller can not give the drilling angle and VAL angle plus the pin to PAP distance the bowlers requires for the lane condition and the measure where the PSA winds up with a DeTerminator , bowlers stats. and what the bower is looking for out of the ball along with any necessary surface adjustments.
What I have brought up I realise is way beyond the novice but is substantially what will give correct ball motion in symmetricals.