As Cheesybox says, it depends... How low I let myself go before I use a med-pack depends on a lot of things.
First is speed: If I'm below normal speed, I want my health higher, because there's greater potential for the enemies to get in two shots for each of my moves.
Second is how well protected I am: I'm referring to both traits and armour here. If I haven't taken any levels in Ironman or Tough as Nails, I want to be extra cautious, although, if I have, say, Malek's Armour, I can afford to be a bit more stingy with med-packs.
Third is depth: If Mancubi or similar enemies are around, you want to keep your health high.
In your case, you were wearing armour that slowed you down, even with the agility mod, and didn't get Hellrunner until right at the end. Power-modded Onyx Armour offers decent protection, (I got through my first Ao100 game with that) but with no Ironman, every shot that hits you is going to hurt more. And at level 30 Ultra-Violence, you'll be encountering some of the more dangerous enemies in the game. In those circumstances, 30% is far too low to walking around with. I'd probably want to keep my health at 50-60% or higher, and even then, it might not be enough and I would still want to be very, very cautious as I'm exploring. (There isn't really a mathematical formula I use; I just weigh my options and the relative risk, and go with a number that feels right)
I recommend taking levels in Ironman next time; it really helps put off the instadeaths. Depending on your playing style, Intuition 2 might also be helpful in terms of knowing when it's safe to let your guard down. (Although Ammochain blocks that)