Wait a minute... About the damage resistance. If you take a Targe Demo as an example, they have 50% fire resistance. They take half damage from flames in other words? If you were to give their equivalent health boost against flames it would basically be double their standard health or 350 HP right (not counting other health boost/drain items and overheal)? So basically... If you want to factor in health boosts to better visualize damage resistance, then you need multiply the the resistance by 2.

Equivalent Health (350) = ( Base Health (175) x Damage Resistance (50%) x 2) + Base Health (175) = Half Damage (from resisted damage type)

In the Heavy's case:

(Base Health (300) x Damage Resistance (20%) x 2) + Base Health (300) = Equivalent Health (420)

If you just do this:

(Base Health (300) x Damage Resistance (20%)) + Base Health (300) = Equivalent Health (360)

Then by that logic:

(Base Health (175) x Damage Resistance (50%)) + Base Health (175) = Equivalent Health (262.5) rounded off to (260)

That doesn't sound right...

This is from the TF2 Wiki:

"A player may take a modified amount of damage due to certain items. These effects stack multiplicatively. "X% damage vulnerability" multiplies that kind of damage by 100% then adding X%, and "Y% damage resistance" multiplies that kind of damage by 100% then subtracting Y%; for example, the Chargin' Targe grants 40% explosive damage resistance, which means all incoming explosive damage is multiplied by 60%."

In other words Targe multiplies damage by 50% or 0.5 or cuts it in half. Effectively giving 100% extra health (double). By that same principle Natascha and the Beast give 40% extra health. This means a full health Heavy has 420 HP (against all damage sources), and an overhealed one has a borderline ridiculous 630 HP, more than a wrangled sentry (plus enemy weapons attacking a Heavy are affected by falloff unlike buildings). But since it doesn't actually increase hitpoints it means each individual hitpoint becomes worth more. A Medic healing a Heavy with the stock Medi Gun effectively heals this kind of heavy at a base heal rate of 241.4 HP per second. 40% more, 33.6 HP per second, the same as a Quick Fix. The Quick Fix heals at 33.61.4 HP per second at minimum, 47.04 HP per second. Because each hitpoint is worth 40% more with 20% damage resistance. It seems you may have unintentionally downplayed the extra health boost a bit. My apologies if I screwed up and got everything wrong or someone else mentioned this before, I was on caffeine at midnight when I wrote this. On a related note I have realised I have issues...