One-quarter inch is just right. BOSS can still fall out, but not at a very fast rate. You don't want the bottles to empty quickly or that removes the challenge. The bottles should last all day. The holes are too large if they don't.
Take empty Gatoraid bottles or small plastic apple juice bottles and drill quarter inch holes in them. Fill with scratch or BOSS and toss them into the run. I have 23 chickens and I toss them five bottles full of BOSS when they're snow bound these days. The trickle of seeds out of the bottles as...