This the way I check piston to valve clearance:
1. degree the cam
2. install the lifters, pushrods & rockers you intend to run
3. adjust valves to the proper clearance
4. turn engine by hand & watch the exhaust valve. Stop at 10 degrees btdc as exhaust is closing. Install & zero a dial indicator on the exhaust valve, and lever the valve open until it gently touches the piston and measure clearance.
5. turn engine to 10 degrees atdc and repeat the process on the intake.
I like to check it with the springs I'm using to duplicate the operating conditions as closely as possible.....