I went through this recently with an engine I'm putting together and the tight spots were about 8-degrees from TDC. The thing is, when calculating how far the piston is down at that point and how much more the valve is open it's a 'wash'--they pretty much cancel each other out and it won't make or break you unless you're really interested in that last thousandth of an inch. You should be more worried about radial clearance. Mine has plenty of vertical clearance but it seems my exhaust valves are a little farther from the intakes than the piston designer had in mind. It's a good thing I can take care of it myself, it's hell to get anything done by a shop in a timely manner, at least around here.