I can tell you what I have done, not sure it is any better than other suggestions.
for no gap staging (motors taped) no vents are needed. So the following only applies to nonelectrinic black powder tape staging
minimum diameter two holes. I usually use a standard cheap notebook paper size hole punch. One on each side, prevents theoretical risk of asymmetric venting tilting the rocket. Probably overkill. this works for 13 mm to 24 mm D, I’d go with three spaced 120 degrees for an E. These holes are place about 1/8 to 1/4“ below the sustainer nozzle position in “nested” position. You need to have at least 1/4 of the sustainer motor sticking out the rear of these sustainer, 1/2” is better, as the motor itself acts as the coupler for minimum diameter birds, at least for mine.
non-minimum diameter you have a lot of options. I extend the motor mount in chimney fashion so it terminates about 1/8 to 1/4” below where the sustainer nozzle will be on boost. you still need to vent the OUTER tube.
you have multiple options, I will describe two.
the EASIEST is just to put the same two hole you would on the minimum diameter, one on each side. Distance doesn’t matter that much, as the gas will have to go all the way up the chimney to illuminate the sustainer nozzle before it can go anywhere else anyway. Just make sure the holes are not occluded by any coupler you set up, and that they are ABOVE the most forward centering ring of the motor mount/chimney.
the most ELEGANT and aerodynamic is to punch two or three holes in EACH of the CENTERING RINGS of the booster. The gas will then go UP the chimney to the sustainer, slip out the 1/8 to 1/4” circumferential gap between the end of the chimney and the sustainer motor, and the can run back DOWN the booster between the body tube and the motor mount/chimney through the holes in the centering rings, ultimately venting out the tail end. This obviously requires a bit of planning, as you need to make the holes before you glue in the mount/chimney. If the rings aren’t wide enough for that size hole, just glue them first onto the mount/chimney, then cut some notches in them before you glue in the mount. For low power you aren’t going to significantly degrade the centering rings with a few small notches. This avoids poking holes which may be less aesthetic and aerodynamic when placed on the side of the body tube.
of, if you are
@neil_w , you poke the holes in the sides and put decals on labeling them “plasma vents” or “tachyon portals”