The piston stop method is more accurate, because when the rod goes over TDC, there is a dwell period where the piston isn't moving but the crank is. And the amount could vary between rod lengths, or strokes. You won't see that with a camera.
You're idling at 20°? Are you talking "including" vacuum advance? You don't mean your initial timing is 20°, with the vacuum canister plugged.
Not too unheard of, especially for big cammed cars. Stock base timing for late 70's 400's was 17-18.
20° of initial is a ton by todays standards, and back when gas was "good" (leaded), most dist. curves were around 12 0r 13, which equates to 24-26 crank, and with 10-12° of initial advance was a decent total timing of 36-38. Those were good numbers for most street strip engs, with either open, or closed chambered heads.
If anyone was running 20° initial timing, then in order to have those total advance numbers, the dist. would only have about 7-8°, which is a pretty lazy curve. But if we're talkin late 70s, it wouldn't surprise me, emissions tuning being as ridiculous as they were. And I guarantee, they weren't programming those cars, for 36° of total advance.
