Checking quench with clay is not going to give you a safe acurate reading. The quench pads are on the out sides of the piston, off center from the pin. With clay on the piston between the quench pad and the edge of the piston it will cause the piston to rock and lean over on that side and the clay will measure thicker than what your actual quench distance is.

On my zero deck 318 with tight fitting KB hypers I could push up on one side of the piston from underneath the block and the piston would come up slightly above deck and if you pushed down on the side of the piston from the top it would go slightly below deck about the same amount. I asked several engine builders the proper way to measure quench and the pretty much all said to measure the stroke, rod length, pin height and deck height and do the math. Only one said to use the clay method and he said tighter than .050 and the piston will hit the head, the others generally said they would run as tight as .030 to .035 on a tight, roughly 4 inch bore motor. Bigger bores need more clearance because the piston can rock more, also short pistons need more clearance as they are prone to rock more and loose fitting forged pistons need more clearance and aluminum rod motors need more clearance. My motor is .040 and runs smooth as glass.