Nice link, explains a lot. Easiest way to determine how much backspacing you can do is with a straight edge about 24 inches long and a ruler. Set the straight edge against the face of the drum so it sticks out on one side and measure 90 degrees side to side of the edge sitting against the drum, so, if the straight edge is laying against the drum and at the tip of the straight edge I have five inches clearance to a wheel well edge or anything inside the well area that is inside the edge, I can have that much backspacing, and if the outside clearance at the same time is three inches anywhere on the inside of the outer wheel well lip, then the outside spacing can be equal to the two sides, but leave a minimum of half an inch each side (inside and outside) for clearance.Be sure to measure this way all the way around the inside, don't want one little spot to touch the tire, it will wear, scrape, or even cut the tire, which is not a good thing.