As a starting point, draw an imaginary line from mid luff through the clew to the deck. Where that 'line' hits the genoa tracks is a good place to start.
Then, under sail, watch your tell tales... when you 'pinch up' into the wind if the upper telltales stall first, move the lead forward. If the lowers stall first move it back. Ideally all three telltales should 'break' at the same time.
In heavier breezes like those you were in it sometimes is a good idea to move the lead aft to intentionally spill some power from the upper part of the sail.
In the case described was your jib partially rolled up? Since the cloth comes off one side of the roll it's not unusual to have the sail behave differently on one tack than the other since the bulk of the furl affects flow differently on each tack.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"