Unfortunately, the main tell tails really don't tell you if the slot is too open or closed (at least in my experience). This is because there is generally going to be flow over the main even if the jib is too loose or tight. The only indicator of too tight a slot is usually backwinding of the main and counter-rotating of the mast. But by that point, you're way off.

Your best bet (as usual) is to find people who are fast and set your boat up like theirs. Otherwise, get the jib breaking even and try to get it to match the main. If the jib starts luffing way before the main, probably try bringing in the jib car.

A good starting point for upwind would probably be between 12 and 16 inches in from the inside of the hull.

sm