The .300 is the thickness of the head between the pushrod hole and the intake port, thus is the amount of meat I have to work with to cut. I wanted to open the pushrod hole up to the point where I have .020 clearance. Since the pushrods are tapered the higher they go, the bigger they are. When I was checking the scrub on the rocker arms, I noticed I got to a point in rolling the engine over by hand that it stopped. I have over .250 of piston to valve clearance, so checked the push rods and it appears I had a couple that were touching.