So, just looking at this again now that I've split it off. The formula looks to be:
HP Base + Sum( floor(0.5*stat1) + floor(0.5*stat2) + floor(0.5*stat3) )
Where stat1, stat2, and stat3 are the stats associated with the first, second, and third active rune respectively.
It's unclear what determines the "HP Base" term, but it appears to be consistent for a given player and vary across players. Casey_ had a base of 168 and Kensegaku had a base of 158.
For MP restoration, everyone gets:
Tenebrae x1: MP Base*2
Tenebrae x2: MP Base*3
Tebenrae x3: MP Base*4
but it's unclear what the "MP Base" value is here, either. Based on our two examples, it is 0.37*HP Base.
I would guess that the "Bases" are determined by weapon skill.