Someone please correct me if I'm wrong, but I don't think that valves necessarily hit pistons at exactly TDC
That is true but the cam info is published for TDC so that is what you work with. If you know valve drop and cam lobe lift at TDC then you can figure out how deep the valve pockets need to be. That is why Ross is asking for the valve drop of the head.
A cam might have 0.200 lobe lift at TDC. With a 1.50 rocker arm you have 0.300 gross valve lift at TDC. Then you need to know your valve drop, head gasket thickness, desired valve to piston clearance and valve lash and you can figure out the depth the valve pocket needs to be.
