I scoff at talks of offset and backspacking. None of that helps you much. It is frustrating that those values are the industry standard. What we need to know is the opposite of backspacing. I've been calling it "frontspacing." What it comes down to is that you need to know how much the total width of your wheels will add up to when bolted to the constant WMS of our axles.
Nobody lists the front spacing because they figure you only need to know backspacing to determine if the wheel will clear the brakes and suspension. I've found that if you know what front spacing you have now, then you can compare it to a new rim and instantly know how much the tire will move in or out from where it is now. With one measurement I can instantly determine how much of a spacer I need or don't need. If you get the tires to be the same width apart you won't have to worry about backspacing at all unless you're also dramatically increasing the width of the rim and tire. Even then I'd rather know front spacing first for comparison.
To find backspacing I put the rim and tire face down on concrete. I then measure from the concrete to the wheel mounting surface. That is the total amount that wheel will add to the axle WMS. Measuring it is the only way to actually know. You can calculate it if you know the backspacing and the total rim width, but that's retarded. First of all most Eagle rims, I know the 5 spoke alloy sport rims with confidence, are 15"x6". That means the space between the beads of the rim is 6". The thickness of the bead itself plus the shape of the tire usually causes the entire wheel to end up about 2" wider than that value. Without the tire the rim is always at least an inch wider than the advertised width. Too many times I've actually seen 15x6 rims called 15x7 because the owner didn't know he was measuring them wrong.
When you go to XJ rims most of them are truly 15"x7" rims. That means there is 7" down in the trough between the beads. The whole rim is actually 8" wide.
If you know the front spacing I can tell you what works and doesn't work. The Eagle Sport alloy rims for instance are about 1-3/4" front spacing. They are hideously recessed into the front fender well and I always add a 1" spacer behind them. On the back axle it is much worse and for those I always add a 2" spacer. Even with the spacers the tire is still well within the fender flare.
My next favorite rim is the Jeep XJ 10 hole alloy rim. That one has about a 2-1/2" front spacing. Basically that means each tire sticks out 3/4" more than a sport rim would, so your total tire width is going to be 1-1/2 wider. For those I do not use a spacer in the front and I use a 1" or a 1.5" spacer in the back. That makes all of my Eagles look much better and gives them all the same total tire widths.
The limit is 4" front spacing. At that point the extreme outside of your front tires will be perfectly flush with the outside of the front fender flares. In the back your fender flares will still stick out past the tires by about an inch. The 15 spoke rims I sold to Phil have that exact front spacing. Those are 15x7" rims that have an extreme width of 8" and the wheel mounting surface is exactly in the middle, which is why the front spacing and the back spacing is exactly 4".
This picture shows Phil's SX4 with the 4" front spacing and no spacers added:
This picture shows my wife driving my Green SW. In that picture those rims have 1" spacers in the front and 1.5" spacers in the back.
This picture shows my Kammback with XJ wire type rims. With those rims you must have spacers or the front wheel will hit the caliper and not rotate. I think pictured you're looking at 2" in the back and 1.5" in the front. I eventually used 1" in the front to get it where I wanted it.
This picture shows 10 hole rims without any spacers and 235/75R15 tires. I had the fender flares on but don't have pictures of it.