please help with resistance calculations

Dearest forum members,

please feel free to LMGTFY'd me for this, but I have tried and was not successful. Im am sure you can answer this. Basically I want to know if this calculation is correct?

AFAIK a fire immune monster has a fire resistance of >99%.
A sunder fire charm sets the resistance of our monster to 95%.
So a Fireball with let's say 1000 dmg (easier to calculate) makes 50 dmg to the monster.
Let's say the sorceress attacking with said Fireball wears a perfect Flickering Flame. -15% fire resistance. Now which is it?
a) The monster has now a resistance of 80% (95 - (100 * 0,15)) and the Fireball does 200 dmg
a) The monster has now a resistance of 80,75% (95 - (95 * 0,15)) and the Fireball does 192,5 dmg

For the sake of the argument and further calculations let's say a) is correct.
Our fire sorc is in a party of two and her companion is a necromancer with Lower Resist lvl 1 (shold be resistance -31%).
Again, which is it?
a) The monster has now a resistance of 65% (80 - (100 * 0,15)) and the Fireball does 350 dmg
a) The monster has now a resistance of 68% (80 - (80 * 0,15)) and the Fireball does 320 dmg

But oh no, the necro gets killed by another monster. Our sorceress equips her merc with Infinity and therefore Conviction lvl 12 with resistances -85%.
Again, the same question (remember the necro is dead), ow which is it?
a) The monster has now a resistance of 10% (95 - (100 * 0,85)) and the Fireball does 900 dmg
a) The monster has now a resistance of 27% (95 - (80 * 0,85)) and the Fireball does 730 dmg

Thank you for your kind replies...

knightfall

Basic underlying rules for immunes:
- Immunes have 100 or more resist.
- Sunders set their resist to 95 no matter what they really started out at (certain sunder-immune exceptions apply..e.g. traps).
- -res on gear stacks additively from that 95.
- -res from Conviction, lower res and Cold Mastery stacks additively at a 1/5 ratio from that same 95.
- -res on gear and from Conviction, lower res and Cold Mastery stack additively with each other.
- Resistance cannot be reduced below -100 no matter what.

So if a 100 dmg Fireball hits an immune with a sunder, it will do 5 dmg (95 res).

Adding -10 res from gear takes the 95 to 85, meaning the same Fireball does 15 dmg.

Adding a 50% Conviction to that adds -10 res (1/5 ratio on immunes), taking the mob to 75 res and causing the Fireball to do 25 dmg.

Adding a 50% lower res to that adds another -10 res (same 1/5 ratio), taking the mob to 65 res, causing the Fireball to do 35 dmg.

(arbitrarily chosen numbers, obviously)

Thank you, Schnorki, that answers the question perfectly!