Your way of measuring the clearance will work it is kind of hard to know if you put too much pressure on the pan causing it to bow up a little. I use some of my daughters play dough. I put a ball of the stuff on the pick-up then I bolt the pan in place using a gasket. Take the pan off and then measure the play dough where it has been flattened. I believe 1/4 of an inch is fine. Any closer and the pump will not be able to suck up enough oil to supply the engine.