Note my self-proclaimed "nautical newbie" status and consider my comments in that light
ISTM "has seen better days" and the too-tight leech is an unusual complaint? I had thought
that when sails aged, became "blown," the problem was generally one of not being able to get the leech tight enough? It hadn't occurred to me the problem was the sail might not have been designed for your boat, as Faster suggests.
In general terms: Drawing a line from mid-way up the luff, through the clew, will show you a reasonable starting-point for your genoa cars. Then you adjust from there, based on whether you need more twist at the top (move cars back) or less (move cars forward). This is determined by observing sail shape and telltales, and behaviour of the helm. It will change depending on tack and wind strength. From the starting point, you should have adequate track, both fore and aft, for tuning. Otherwise, as Faster suggests, perhaps that sail wasn't really designed for your boat?
How does the sail's luff look? Do you have, can you get, adequate forestay and halyard tension?