ok, let me explain in more detail.
Your suspension is intended to be directed down through the wheel and is balanced with an expected location of the center of the rim. The SUSPENSION dictates the optimum offset, not the rim or the tire. If you were going to run a 3" spare doughnut (assuming it's a 25" tire unlike the one in the trunk) and asked what offset would be best, I'd say 53mm. If you wanted to run a 12" wide drag slick, you'd get the same answer. Course, neither of those work well with the stock wrx suspension for other reasons but they are unrelated to offset.
The reason there's a range is nothing to do with rim width. A 45mm offset on an 8" wide wheel is no better than on a 6.5" wheel. They're both bad. 48mm is just an engineering judgment, a "fudge factor". Commonly, people tell you to be within 5mm of stock, that's all 48mm is. It's not some magic number that has anything to do with wheel width.
Now, since you really don't want to worsen the handling of your car (I'm assuming you value handling), you'll want to keep the center point of your wheel fairly close to stock. This, in effect, limits your wheel width because there is only so much room between the hub and the fender. Even swinging a 48mm offset (already 5 off), an 8" rim comes in 14 mm more (15mm is about all you've got to work with). Any wider and I'd a) have way too small an offset or b) rub.
Does this make more sense?
04 STi 16G powered lagless fun
04 WRX: SOLD