When in doubt, RTA. Generally the best method for timing.

i only switched to SDA timing because caveman was using it

My timings have always been weird and varied but that's mostly because I'm newer to speedrunning and my times probably aren't really relevant enough to question exactly when it starts and ends. But I did always end on the Nahatomb fadeout, just as his crystal (next to the health bar) breaks, and this seemed most logical to me, seeing as you can still be hurt and/or die before then.