OP, just based on your wheel sizes I'm thinking 80 mm rears are in your future.
Or you could bow the 50mm overs out to 80 maybe if you don't care how crazy it looked.
So basically get the tires mounted on the wheels and cut the stock sheet metal out, then pull the springs out of the coilovers.
Then drop the rear of the car to the max compression on the shocks.
See if that's low enough for you, and then tape the over fenders on and start attaching so that they cover the wheel and fit how you like. This is where the bowing of the overs comes in.
If worst comes to worst, you can always attach flares to your overs...
Simple math: there's a guy in the max tire thread on 12" wide +20 offset and 335/30/18, and 50mm overs, but his car isn't that low. And he has an s13 hatch.
Well you're going to poke an extra 42 mm just in negative offset and spacer, and then another 6mm from being 1/2" wider. So if he's on 50mm overs and barely fitting, you need 100mm overs for those wheels.