If you increase max health, the percentage of current health stays the same. Meaning if you have 100 max health but only have 50 current health, and then you gain 100 max health (like from Nihilus’s Annihilate), you stay at that 50%, so you’d be at 100 health of 200.
It’s anecdotal, but I think that’s true because I use the Nihilus strategy in the Jedi assault battles. My Nihilus has 34k health. If I annihilate one guy that has 174k health, he stays at whatever percentage health he had before the annihilate. I say this because he’s been his for 40k afterward and not been killed.