My rule of thumb is directed at sailing. Wind, sea conditions, colder temperatures and what the crew get up to when the skipper is off watch being variables.
On long passages I think it is well worth using the engine for larger sail changes,also it clears the exhaust manifold.
When anchoring I always start the engine rather than use battery power for the windlass.
I think I have that Excel from when you added the solar panels, but it would be a really a useful reference if you would post it.