Piston is chasing the exhaust valve as it closes, and the smallest clearance is something like 5-10 deg BTDC. The intake is opening before the piston starts down, so its clearance is the tightest at 5-10 ATDC. Unless I got those backwards
Anyhow, the rule of thumb I usually see is around .080-.100 intake, .120-.140 exhaust - but if you're brave, you can run it closer until you start seeing witness marks on the piston top!