The percentage is as it says on wiki and added to your current chance to crit.
So when you finish your Berserk one you'll get 1/5 of your % missing hp converted to crit chance added to your 10% from items and base warrior crit rate (5%? 10%? I forget).
Rank 1: 1/5 of missing percentage of health as crit chance (missing 60% health = +12% crit)
Rank 2: 1/4 of missing percentage of health as crit chance (missing 60% health = +15% crit)
Rank 3: 1/3 of missing percentage of health as crit chance (missing 60% health = +20% crit)
Rank 4: 1/2 of missing percentage of health as crit chance (missing 60% health = +30% crit)
It's good when grinding since you spend more time floating at reduced health; especially if you do it on purpose for the crit chance.
I was sad to not get rank 3 and 4 on gwaine because the crit chance would be fun to parse at rank 4. I only have rank 2 done.
I looked back at a parse of 159 mobs in a row that I could crit against. My crit rate was parsed at 33.05%. Rank 4 would likely put me over 40% crit in a typical duo or exp situation.