Generally I look at having 7 or fewer OL before converting to OL, Two TEs or fewer, 4 or fewer WRs and 2 RBs and 6 or fewer DL. Nothing converts well to QB and I never needed to convert to LB or DB before. Typically get +WE every time.
It also takes into effect the amounts of the position being converted from but I do not have observations in that regard.
Anyone remember the good ol days when you could change to any position as often as you like. It was great for turning junk sim recruits into gods. Ha.