Comp has a lobe catalog on their website that shows tappet lift at TDC for each lobe. An aggressive roller might have 0.150 tappet lift at TDC. Multiply that by your rocker arm ratio and that tells you the valve lift at TDC. If you have 1.70 rockers then the valve lift at TDC will be 0.255. Subtract that from your valve drop and you'll know what your valve to piston clearance will be with the new cam.
