It doesnt actually matter where the wheel width is taken from. Measure the complete width of the wheel. I usually find the actuall overall width is half an inch more either side than its official width so a 7 inch wheel would measure 8 inches. I use two sliding squares with the ruler part clamped together in the middle. This should get you the full width of the wheel. Now work out half this amount. Bare in mind wheel widths are usually measure in J ie inches. You need it in mm. Using your sliding squares again mark off this half way point. Now measure from the back face of the centre of the wheel to this middlepoint you have marked. This is your offset in mm.