There's an AP breakpoint for that. Mendenbarr calculated it to be ~136k AP for the default BiS profile in SimC using the following equation:
Using SR for single-target is a DPS increase under the right circumstances, but determining that on the fly can sometimes be tricky with mastery's fluctuation due to the 2P. If you're concerned about it, track your AP and try to abide by it.
(((main-hand weapon damage) + (2.4 *(X/ 14)))*1.3*1.15*1.04*0.68+(60000+1.2X)*1.15*1.05=(499+0.8487X)* 1.10*1.05*1.15*mastery