I would say that 30-40% is good. In college every year there are some catchers who throw out more than 50% which is exceptional!
I would agree with the other statements that college coaches and scouts aren't looking at your %, they're looking more at your pop time. HOWEVER, your current coach will be worried about the %...if that % gets too low then you won't be playing ver much and no scouts will get to see you play!
If you worry about throwing runners out and put pop times secondary, everything will take care of itself.