That's how you know you're on the comp stroke is by the air forced out of the vacant plug hole. If the piston was then at the top of the stroke as correlated by a piece of dowel rod or screwdriver to indicate peak upward travel, in combination with seeing the timing scribe very near 0 on the index, then that piston is at TDC on the comp stroke. From there it comes down to how far the distributor gear may have rotated as the distributor was inserted, and how closely aligned the rotor was under one of the holes on the distributor cap. That would be where the #8 plug wires goes.