I have been playing around with different wheel back spacing and tyre sizes.
I am trying to fill the rear fenders in as much as possible without rubbing and without the wheel being setback too far in the opening. I also only wanted a mind flare on the rear instead of going for the larger 4" flare.
The first attempt was with a 18x11, 4.75"BS rim fitted with a 305/40r18 tyre. This gave plenty of clearance between the tyre and chassis however it put the outer side of the rim and tyre right on the lip of the fender. With a diameter of 27.7" it was also too big.
Next was my previous 18x9.5, 5.25"BS rim fitted with a 275/40r18 tyre. With this I made up a 1.5" spacer to mimic a 18x11 rim with a 5.25" backspace. The tyre was also an inch less in diameter at 26.7".
This set-up gives sufficient clearance between the tyre and chassis and should be ok for the outer tyre and fender lip.
The maximum backspace I could go is 5.5", however I think its cutting it a bit fine for chassis clearance. Using the measurements from the 9.5" rim I should be able to go 18x11 (12" from lip to lip) rim with a 5.25" backspace fitted with a 315/35r18 (26.7" Dia) tyre.