How does scaling work with +addMD and +trueMD?

So I'm just going to go straight into an example with zap and how I think it works, someone please tell me if it's right or wrong :

So Zap has a base damage (Base) of 20 on the hit and Base = 1 on burn; the scaling for the hit = 1.0, burn = 0.5.

So say I have a blue book (5 MD), +4 AddMD in jewelry, and 0.3 MPBonus.

So the hit damage would be: (Base + 5MD*SCL)*(1+MPB(SCL)) + AddMD*SCL ? =>

=> (20+5*(1.0))*(1 + 0.3*(1.0)) + 4*(1.0) = 36.5 damage before enemy MR; burn = (1+2.5)*(1+0.15) + 2 = 6.025 in 1 sec.

But is +trueMD still subject to scaling and the "modified/scaled" damage just then ignores the MR of enemy?

like if I had the previous formulas and damage numbers with +4 trueMD and not addMD. Assume enemy MR is 0.2.

Would I just have (on direct hit of opponent) 32.5*(1-0.2) + 4 ; and (4.025*(1-0.2) + 2) or (4.025*(1-0.2) + 4) for burn?

I just wanna know if mainly my calculations are correct and if trueMD is subject to scaling or if it always deals flat +x damage.