Blue Rocks and BUs

Hello,
I was just curious as to how blue rocks stack with essences of celerity, and the capping involved. Do they stack by being multiplied together (i.e. 20% IMS from BU * 25% IMS = 50 % IMS which is then capped at 33%), or in some other way? Can the blue rock max out everything (IMS, IAS, Activation Time) when stacked with an essence? Also, I assume these bonuses all are capped at 33%. Am I wrong on this?

I suppose my question in a nutshell is: My team pops cons. Does using a redrock provide me any benefits (other than lasting 10 mins longer) over a blue rock?

Thanks guys, this is a HUGE help .

http://wiki.guildwars.com/wiki/Effect_stacking

That page explains it. Celerity and a Blue Rock will result in IAS and IMS of +33%, 25% reduced activation time, and 20% faster recharge. Red rock will just increase the duration to equal that of a Celerity. It will provide no other mechanical benefit.

-Percantages multiply, as you suggest. 1.2*1.25=1.33 (considering the cap).
-I would expect Blue Rock Candy + Essences to max IAS, IMS, and decreased activation time.
-There is effectively no benefit between using a Blue Rock and using a Red Rock given that you are also under the effects of an Essence. In fact, you could use a Green Rock and still max everything.