Normally with my 4" zipperless designs, I drill eight 3/8" vent holes equidistant around the perimeter of the baffle plate. But in this case, I didn't want any of the holes directly above either of the motor tubes. The half moon baffles keep burning particles from passing through, but there's still a lot of heat. So I decided on six 1/2" vent holes arranged so that they're to the sides of the motor tubes. Out of curiosity, I figured the area of 3/8" and 1/2" circles. Six 1/2" holes have a larger total area than eight 3/8" holes.
Using a template I made with the template from Open Rocket, I marked the positions of the vent holes on the plate. I must have been drilling too fast, it kept catching the plywood and spinning it out of my hand. Made me cuss. It also chewed it up a bit. But I got the holes drilled, finally.