I feel like I should already know this, but I'm trying to nail down top dead center to check my distributor alignment. I think my long-standing crank but no start condition may have been caused by the disintegration of the plastic distributor drive gear (since replaced by a new gear.) The problem is that the piston is at top dead center twice during the 4-strokes of the engine. On one of the TDC approaches, the intake and exhaust valves open, flanking the TDC event. Is this when the rotor should point to the #1 spark plug wire in the cap, or is it during the other TDC event when both lifters are on the base circle of the cam?

You want the other TDC when the valves are closed.
When the valves are slightly open at TDC that's the end of the exhaust stroke and the rotor should be pointing 180 degrees away from the #1 post.

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OK thank you, that's what I reckoned, but wasn't exactly sure. I checked the rotor and it seemed like it was lined up correctly. But then I looked at the valves operating in that position, and it seemed backward. When the distributor drive gear failed, it somehow ended up 180 degrees off. Hopefully it will start once I get it straightened out.