275/60/15 woes

Here is the thing , they say the backspace is 4 , 4.25 or 4.5 , but is it actually that. I have bought wheels from Jegs and Summit , because Specialty Wheel didn't offer 4.25.
I tried 4.5 , hit the spring with 275's , actual backspace was 4 5/8 and 4 11/16. So I ordered a 4 inch backspace , got 4 1/4 and 4 3/8. Swapped wheels side to side to see what worked best. My Duster has a good 1/8 to 1/4 difference , side to side. Sure would be nice if they followed their own specs.

Frank