Mass Effect tracks cooldowns differently than in most games. Rather than mapping increases / decreases to the base cooldown of the power (where a 100% reduction would be no cooldown), it increases / decreases the number of uses per second.
The Theory is as follows:
Assuming a 5 second cooldown at +100% cooldown would be twice every 5 seconds, or 2.5 seconds, and +200% would be 1.67 seconds.
Reverse would work the same way. -100% cooldowns means once every 10 seconds, and -200% cooldowns means 15 seconds.
I decided to pull some numbers from my finished ME3 playthrough save, my Vanguard.
Notes on the Numbers:
- I respeced Shepard, clearing all skills.
- I still had one of the research options for -5% power cooldown.
- All testing took place on the Normandy, which meant no power cooldown gear was applied. I switched my gear to damage or shield bonuses instead, just to be safe.
I have 3 setups for Weight Cooldown Bonuses of -200%, 0%, and 200%.
And here were my numbers. I'm using the skill "Pull".
With the 0% loadout:
With the +200% loadout:
With the -200% loadout:
Other Values: -1% total CDR: 4.07; 0% total CDR: 4.00
Pull has a base cooldown of 4.00. Following our Hypothesis, -200% cooldowns would give a cooldown of 12.00 seconds. Our 5% CDR has a cooldown of 11.8! That's pretty close to the expected!
Math confirms that this is, in fact, correct: 4 (1.00 + 1.95) = 1 / 11.8!
Additional bonuses, like rank 2 of the ability, stack additively. 1 / 4 (1.00 + 1.7) = 1 / 10.8!
So this works fine and dandy when your malus is a penalty, but what about when you have a bonus? Bonuses are a bit easier - simply divide the cooldown by the bonus %, converted to a decimal.
4 / (1+2.05) (from our 205% weight + research bonus) = 1.31
4 / (1+2.30) (from rank 2) = 1.21
If you have a net bonus, formula is:
[Old Cooldown] / (1+ [Percent Bonus As Decimal]) = [New Cooldown]
If you have a net penalty, formula is:
[Old Cooldown] * (1+ [Percent Bonus As Decimal]) = [New Cooldown]
It's late, so if someone wants to check my math / make a pretty picture with the formula / math markup, please, be my guest.