When I dust, I will usually just dust all of them very lightly. You don't want your crickets, roaches, worms, etc., to be fluffy white, you just want them to be a little paler. Or maybe have just a few speckles of powder, depending on the stickiness of your supplement brand.
Or (and this is for when he's older) if I'm feeding an adult male who doesn't eat a lot or every day, I may take the big roach he's due and dunk the butt in some calcium, so there's a fair amount on there. Again, not fluffy white caked on, but more than usual. And then I know he's getting the equivalent amount of powder but on one feeder, instead of 10, or whatever a baby is eating.