The only finisher that can be used at less than 5 CP's is Savage Roar really, otherwise you're just wasting Energy. It's fine if you can't use FB very often in a fight, keeping SR and Rip up has a much higher priority than FB.
The "golden" rule is if you have 5 CP's and still 10+ seconds left on both Rip and SR, you can FB. Probably won't happen very often, and some people will prefer using that energy to Shred beyond 5 CP's anyway. Since the minor nerf on RnT, FB isn't a guaranteed crit anymore and some people feel if it doesn't crit it isn't worth the energy.
Only thing I would say here, if it's not already what you do, is to burn your first Berserk right after the initial SR. Makes it much easier to get the rest of your rotation going. The short initial SR won't be a problem as you will build points fast, put Rip up, 5 more CP's and another SR just before Berserk ends.i run in farie fire, mangle> SR> rake>shred> 5 combo pt rip> refresh what i need to from there