So +20/20 = 1. Where did you get the info before you ordered these?
Now do you mean your offset of distance from the hub mounting to the center of the wheel is 20mm? If so for a regular body your rear 11" is good, check the front, it is questionable, it sits about where the 9.5" was but it has that extra 1/2" (half of the extra 1" width), on the inside and sometimes that is too close to the knuckle.