It's impossible to say exactly where your shaft should be pointed for any sidespin shot unless you're the shooter and are taking into account all the things that influence the shot: distance, speed, amount of sidespin, butt elevation, ball/cloth conditions, etc. These all factor into the amounts of squirt and swerve, which (as you know) counteract each other in varying amounts depending on the varying factors....what about 'parallel' to the shaft deflection angle I refered to...?
For shots where swerve dominates over squirt, aligning the cue somewhere in the squirt direction is right (I think this is what you suggested).
For shots where squirt dominates over swerve, aligning the cue somewhere in the other direction is right.
For shots where squirt and swerve cancel each other (my example above), aligning the cue parallel to the intended cue ball path is right.
pj
chgo