Minimum valve to piston clearance occurs at about 10° BTDC or ATDC as the intake valve is opening and before the piston moves away from TDC. Modeling clay is put on the valve reliefs about 3/16 thick. Install light checking springs available at a hardware store. Install the head with the piston down 1/2" to 3/4". Gasket is not required as a gasket will increase your clearance. Carefully turn the engine through 2 revolutions. Then remove the head and cut the modeling clay from center of the engine to the outside, and pry half off. The remainder can be measured with a caliper to determine the clearance. The outside edge of the valve head to the valve pocket can be measured at the same time.
