The defensive spectrum that wis uses in SLB is SS,2B,CF,3B,RF,LF,1B.
An easy way I imagine to look at is, are the 'recs' for the position you have him playing higher than what his projections are? So, if you have one of those CF/2B guys, I'd assume playing him in CF would increase his FLD/RNG quicker, playing him at 2nd would increase his arm quicker.
Preferably you'd want to play him at a postion where all four rating are higher than his proj's.