cam for flat tops with no valve reliefs

You want the clay to be dented. If it doesn't dent, you ethier didn't put enuff down or the valve is just miles away. You want it to be dented so you can measure the thickness at the thinest point. That would be your clearance.

Remove the cylinder head.
Put down a 1/2 inch of modeling clay.
rotate engine 2 full turns
Clay gets dented
cut clay in half and measure the thickness of the clay at the thinest point

re-read post #3, BJR makes a point.