Loadout Health

The Health attribute you see on your Loadout screen is a combination of your Hero Health and Armour Health with a bonus for Armour Mastery permanent upgrade together with any Health % bonuses (e.g. Frostina at 20 gives all heroes +5% Health; or some heroes have it in their skill tree):

Loadout Health = Hero Health + Armour Health * (1 + Armour Mastery / 100) * (1 + Health % Bonuses/100)

(The Hero Health you see on the Hero screen will already includes the Health % bonus)

Loadout Health for a run can be boosted by Health from Mystery Crates - the amount shown (e.g. 750 Health) gets a bonus from Armour Mastery (probably because the Health awarded is added on to the Armour Health boost) but not from the Health % bonuses:

Actual Health boost = Nominal Health Boost * (1 + Armour Mastery/100)

e.g. I currently have 15% Armour Mastery and the 750 Health gives me a 863 boost (863 = 750 * 1.15 rounded down)

You can estimate the Loadout Health (and Loadout Attack) using the Hero Stat Simulator