you are actually delaying an RB for CS. with the posted rotation you end up losing 1/20th of a CS, and 1/2 of an RB every rotation. you also run the risk of "losing" slam procs because you go 3BT's with out a slam GCD, however this can also be a good thing because if you use CS but are unable to RB, you are more likely to be able to slam on the 2nd GCD to fill the GCD table.
If you pause 0.5 at 19.5 to use CS at 20.0, you lose 5/3 BT's, and 1/3 RB.
if you choose to use RB->BT->Slam on the first CS of of the fight:
if you have no pause of the RB at 19.5, you lose 1 full BT and 1/20 CS every rotation, you also run the risk of not filling the the 3 GCD's during CS due to one of them being a slam GCD
If you pause 0.5 at 19.5 then BT-RB-BT, you lose 3/4 RB and 1/6 BT
If you pause 0.5 at 19.5 then RB-BT-Slam, you lose 1/2 RB and 11/6 BT

