I have been starting the at the first frame that is not a black screen and ending it when the little fireworks go off. Is this right?

My intuition was: Timing starts upon tapping/clicking to start the first level and ends when the final glass turns happy.

But timing from the first non-black frame sounds like it will only remove a load inconsistency, so it seems like a good idea

Yeah, agree with @IwerSonsch

Timer starts upon tapping to start the level