Well, I've verified that Health / 5 for the ancient golem varies with time, same as health -- I'm betting it's some flat percentage. However, my experimentations were not exact, and I do not have perfect data for you.
What I can tell you is that Health and mana regen at approximately .5 second "tics". At the beginning of the game, the ancient golem ticks 2 four times, and then 3 once, for an approximate regen of 11 / 2.5 seconds, or ~22 health / 5. At the end of my 40 minute practice game, the ancient golem was healing at a rate of alternating 3's and 4's, or an approximate regen of 35 health / 5.
So it doesn't scale that much, but it does scale. If someone has better data, or better metrics, please let me know. I'm going to hypothesize that the golem's true regeneration is somewhere on the order of 1.5% of max health. Hopefully, that's close enough to the real thing that you can complete your simulation.