The .020" is a VARIABLE, not the actual clearance there. What I meant was that different rods, piston heights, deck variance, even the side to side placing of the combustion chamber in head casting to alter location of piston valve cutouts all added together will affect the number by that much to alter it one way or the other, and just a guess. You can only get exact figure by measuring YOUR engine, no others will be exactly the same as yours.
One could use light race checking springs and run piston to TDC and then rotate cam around to check the space there using a dial indicator. Save old head gasket as a fitting shim to space head correctly away from block.