The standard system calculates: Start - Pocket = Aim.
Adjustment Cheat Sheet
Rail Speed & Friction
If the rails are slow, you typically need to aim slightly further "up" the rail (add +0.5 to target). If rails are brand new and fast, you may need to shorten the aim.
The Effect of English (Spin)
Running English (following the direction of the shot) will lengthen the shot path. Reverse English will shorten the path. Use center ball for the base calculation.