The contact or tight spot of valve to piston clearance will occur around 10 degrees before or after tdc as those are the areas where the pistons and valves are chasing each other----

Also the diameter of the valve to valve notch/position is critical------I like to use modeling clay on piston top and turn crank over two complete revolutions to sqush the model clay to shape-then with exacto or razor blade you can cut sections of clay away to measure or visually check the clearance---this will also show sqush area, dome to chamber, etc-----

I've just been working on 630 BBC for monster truck bash-should of taken pictures-----also of crank/rod/counterweight areas------