Amount of HP lost (variable) when damage is dealt formula?

1st of all all stats are variables NO health extension

I was always going for displaying current damage dealt as how much HP damaged object lost

But now that i think of it
How do i display instead HP lost i mean is there formula for that?

I do not want to display damage my player dealt i want to display how much HP damaged target lost

If any1 wonder why then imagine all that damage reduction + armor + whatever resistance combined need to be added to formula

I think it would be easier to just display how much HP target lost if that is even possible

Save old HP to any variable
Damage enemy

Change text of TextObject set to Enemy.OldHP - Enemy.HP


I was afraid someone gonna write that
That was my initial idea
Problem with this approach is that i did more or less same exact thing to trigger some action if player moved any direction some steps
On camera position change
And it requires correct events order to work so i was looking for other method