Biggest tires for mini tubbed Duster

Well I took a closer look and the previous owner who installed the wheel tubes left about a 1" lip out from the frame, AND he didn't install them straight up, they actually angle towards the fender giving me even less room at the top, so it looks like I'll probably redue the tubs since they were done very sloppy anyway. But from the frame to where I have the lip trimmed too, it's looking like roughly 16.5" of clearance. So a 15.4 sidewall width on a 15x12 rim should squeak in with 1/2" per side with the right offset, or I could use a 15x10 rim and get a bit more clearance, but how much?