20x8.5" rim w/ 225/35/20 rubber, what backspacing

Ok, ive got a maz that lays frame right now on the stocker 14" with big tires. What i want to know is with a 20x8.5" rim and a tire that is 225/35/20, what backspacing should i get so that they will tuck fine still.

all the other firewall work and fender work im not concearned about, just want to know the proper back spacing, is is about 7" ??

thanks

taylor