Anyone know where to measure? I get confused between offset and backspace.

If you know your backspace, you can figure out your offset easily.

Just measure the width of the wheel and divide that by 2. Now, take that number, and subtract your backspacing. That should give you your offset.

Basically, the offset is the distance between the center of the wheel and the hub mounting surface. Once you know that, you should be able to figure out how to measure it. Long guidline short: after you measure the width, just use a straight edge on the back of thew wheel and measure.