Some still freak out when they hear 42 lbs for poly. Jack Sock plays all poly at 38 lbs in mid+ and has a much faster swingspeed than anyone on TT. Daniel Nestor is playing natural gut mains/poly crosses at 40/38 lbs, and he too has a much faster swingspeed than anyone here.
I play multi mains in the 50-54 range with TF RuffCode poly cross -4 lbs below mains. 52/48 works well.
Try BC around 48/46 and adjust up or down until you reach nirvana. Full poly at 48/46 will play plenty firm and generate loads of spin.