Measure your 10% snack maximum based on how much feed you are giving them, make no allowance for what they are getting free ranging. My flock is below, about 55 birds most of the time. (actually, I need to edit my sig, five hatched this AM) Anyhow, in theory, I should be feeding around 14# per day, based on flock size and the "thumb rule of 1/4# per bird per day" (which doesn't account for hatchlings well, but whatever). I actually feed 10# per day. So my treats, if any, would be 1# or less for the whole flock.
The other thing to keep in mind when free ranging is WHAT they are free ranging on. If their only choice of plant material is tall fescue, the free ranging itself will contribute to an imbalanced diet. Same if its a field of sunflowers, or a shady sput onder the grape arbors while they are dropping fruits, or next to the blueberry hedge...