One mark, but you can see it on compression (when the valves are closed) OR during overlap. So it should be "actually at tdc ON THE COMPRESSION STROKE". As for the feeler gauges, try to find ones with narrower, tapered ends, and bend the one you need about 45 degrees, 3/4" from the end. It will slip in much easier that way. It's nesrly impossible to do with the standard width feeler gauges.