I don't understand Cast On Crit

Hello guys!

I do understand how CoC works: hit with a skill if it's a crit you cast the spell linked. It appears so simple, but reading around I saw that there's a lot of math involved in this...

I want to create a CoC Barrage (or Blast Rain) build, but I started reading around about 33ms of server-don't-know-what, aps on bow, actual crit chance, windup time (40% of the skill time is useless) and things like that.

Can anyone explain like I'm 5 where I've to start? Put that as simple as you can please, with no cooldown reduction or any +1 arrow... just barrage - coc - 2 spells :D

Thanks for your time!