what offset to tuck 20x10 in rear on my widebody eclipse in rear?

just like topic says, i am wanting to put a 20x10 in the rear, i have been running a 20x8.5 with a 40 mm offset and i have used a 1/2 in spacer to push it out alittle to tuck 20s. i still have like 1.5 or 1.75 before the quarter. i want to be able to lowwer it with the airride. any suggestions? i was thinking like a pos 14 or so. i want the wheel and tire combo to sit on the inboard about where it is now with the spacer. is that possible? thanks in advance and sorry if this is confusing. btw the fender is about 3 inches bigger than stock.thanks ben