Is 'frame drop' a common term for latency? Dropping frames means something else entirely to me.

Anyway: you can calculate things for your particular rendering framework all you want, that only tells you when that software considers something visible, and you're still missing the extra frames the GPU driver or the monitor itself might add.

In the end total latency can ime only be measured accurately by measuring both the time when you send your 'show' command and measuring when the thing actually becomes visible (photodiode on the monitor, or one of those expensive monitors which can e.g. output a TTL signal upon seeing a certain pixel value).