What's not to understand? You add a spacer, so the float goes negative and you can measure how much the housing lacks fitting where it should. Subtract the measurement from the thickness of the spacer to get the actual end float. Add shims as required to bring the float within range. The only tricky part is that you need a spacer that is flat and parallel, most hardware store washers aren't flat enough.
Or, assuming you haven't changed the bushing, just put it together with a single gasket and see if you can move the gear at all. If you can wiggle the gear, it's close enough.