The only reason I can think why the crownboard might need a minimum (or maybe maximum) size feed hole is to “fit” a particular feeder entrance.
Within reason, I can’t see any need for precision. Like you say, for 95% of the year it’s blocked off. I only use the feeder hole for fondant (sometimes) or a round rapid or contact feeder. Most of my bulk feeds are with English feeder that just needs to line up with the hole, size or shape is not critical. Miller or Ashworth feeders dispense with crown boards entirely apart from as a lid for the feeder. So location and size may be more about your existing feeder requirements.
Within reason, I can’t see any need for precision. Like you say, for 95% of the year it’s blocked off. I only use the feeder hole for fondant (sometimes) or a round rapid or contact feeder. Most of my bulk feeds are with English feeder that just needs to line up with the hole, size or shape is not critical. Miller or Ashworth feeders dispense with crown boards entirely apart from as a lid for the feeder. So location and size may be more about your existing feeder requirements.