Backspacing is simply the distance you have between the hub and the back of the inside of the wheel. The only time you want to pay attention to that is when you have huge calipers that take up a lot of space. Stock calipers are fairly shallow, and allow to run a fairly small amount of back spacing. Calipers with opposing pistons usually require a lot of backspacing to clear the wheel.
Example: I've been running Cobra calipers, so I was able to use any of the Ford wheels, as long as they were 17 inches. When I switched over to Brembo setup, I had to use either Cobra R wheel, or 98 Cobra wheel, because those are the only two which have that much space in the back of the wheel to clear the caliper.
Edit: Also, just reread you post, and I think you are talking about offset. Offset is very simple really. 0 offset means that the wheel is centered perfectly right on the hub, which means that if you draw a line right rought the surface of the hub half of the wheel is going to be on one side and the other half is going to be on the other side. When you add offset or subtract offset, all you do is move the wheel in or out of the fenderwell. Positive offset moves it out, and negative offset tucks the wheel inward. If you really want to figure out how to make your rear wheels flush with custom offset, you need to measure how much space there is between the hub and the edge of the fender. Then you can take the width of the wheel you are planning on using, devide it in half, subtract the half of the width from the distance you have from hum to fender, and the difference you get is your offset. If you used inches to measure, convert it to millimeters, because that's how manufacturers measure offset. Make sure to measure both directions, so you don't have interference inside of the fender either.
