Google document with gathered numbers. Green values are calculated using the "normal" damage value as a base. Black values have been manually measured to the exact hit point. Blue values have been estimated by inflicting damage and multiplying by the healthbar.
That enemy health bar is 200 pixels long. 100% of the enemy's health. You bring your strongest weapons with your largest amount of bonus. It does 400 damage. There's 100 pixels left on the bar, 50% of the enemy's health. All you need to do now is upscale the damage from your strongest weapon. 400 * 100 / 50 = 800 health in total. This is accurate to within 2% because 400 damage being 100 pixels is accurate to within 1% (0.5% in either direction). A single screenshot of your strongest attack after hitting an undamaged enemy is all you need to gather some pretty accurate data.
You can also run through the level, recording. Hit the enemy a bunch and add up all the damage you did until the enemy only has one hit left before they died. Doing this, you can get a very accurate reading on the enemy's total health.
It's a lot of work, to be sure, to gather it for every enemy at every depth at every amount of party member; but at least it's about to become a lot easier than previously.
Do we know what happens to the UI when there are many enemies on the screen? The only screen shot I've seen shows the Royal Jelly's health bar, but not the health bars of the mini jellies or polyps that would also be in that fight.
I recently started a project to collect enemy health numbers, by laboriously recording hits with various weapons. For example, a fire polyp, on depth 24, when solo, has somewhere between 733 and 736 health. Enemy health bars should make this kind of calculation dramatically easier. So I'm putting my project on hiatus.