40% is pretty low, and you shouldn't use FG%. Use eFG%, which is easy to calculate but you have to do it yourself. It just accounts for the extra point you get for making a three pointer:
(FGM + (0.5)3PM) / FGA
Even better is total shooting percentage, which includes the rate at which a player gets to the foul line and how often he converts when he gets there. I don't remember formula off the top of my head.
But I agree with the overall point, I do adjust my distro based on how players are performing, and tend to increase distro on the ones who are scoring efficiently. Be wary of small sample sizes though, I don't look at these numbers until at least after the non-conference slate. (some would tell you a whole season is too small of a sample size, but for me imperfect data is better than no data)