10 speed rear derailleur to shift 11-40 cassette on 3x10 This is a switch from 9 speed to 10 speed.... I have a 10 speed FSA carbon crankset 52-39-30 I wish to run cassette 11-40 10 speed...  I will size chain length to shift 39-40 and 52-30 no interest in running big big.   Suggestions for rear derailleur and shifter.   I have read that Shimano SGS long cage with B screw all the way in will shift to 40 cog.   Thoughts?
For this setup you would need a derailleur with a capacity of (52-30)+(40-11) = 51 teeth.  This page states that the SGS has a total capacity of 45 teeth. That's seems quite a bit below what you are trying to do, even if you don't plan on running big/big.