checking valve to piston clearance

probably didn't explain very well - I don't have a cam at the moment and want to check how far the valves can be opened (by hand with checking springs in place of regular valvesprings) before they hit the piston so that when I order a cam I know I won't have any problems. Obviously when the piston is at or near BDC there will be no problems and it's only around TDC and/or max valve lift that I need to be concerned about. I seem to recall reading that if you checked from 20 degrees before TDC to 10 degrees after 10 TDC that would cover the area in which interference was most likely

Can anyone confirm/deny this?