Should be a 5 inch backspace. Measured wheel width doesn't take into account the 0.5" lip on either side of the rim(so the true width is 10"). Backspacing does take it into account. Plus a 17x8 in 4.75 will just clear the rear guards when the car is lowered, so to get a 9" in there you'd have to go to 5" backspace.
Interesting you mention the rim touching the UCA, as with my 17x8 4.75, the rims are about 5 mil off the UCA, and judging by a calculator I just used you have 5mm less clearance inside.
I'm still surprised you got 9's on the back and the car that low. Do the wheels touch the guard on compression?