Sorry @nsaspook, after reading your linked post, still not sure; the "critical timing points" are actually inside the ISR? Do you always measure lattency of anyone against the very first, isn't it?I usually 'Instrument' several critical timing points in the interrupt code.
https://forum.allaboutcircuits.com/threads/project-ideas.143685/page-2#post-1330315
Now that I think of it, they do not need to be inside necessarily, right?