I would be interested how others size it out.

Jay:
I use a system similar to yours - grabbing a bulkhead pattern from the mold pattern at a nearby station, then marking for trimming with a light on the other side. (LED lights are really nice; few worries with heat...)

In the past few years, most of my bulkhead installs have been in Mariner glass boats, so I start with an undersize cardboard pattern and then use scraps and a hot glue gun to glue tabs around the perimeter, to generate a pattern.
I set the glass bulkhead in place (sometimes taped to a support of some kind if it's 'loose') and then use pea-sized dots of epoxy putty (SmartWood or similar - the stuff that looks like a Tootsie roll in the package) to set the bulkhead so it doesn't move during filleting.
Then it's the Ziploc bag with the cut corner to lay in the epoxy filleting blend, and a plastic tool to shape the fillet. A light touch with a dry chip brush with the bristles cut short (in a curve) smooths out the fillet. Like you, I usually do the other side later, but I usually have a good look at the other side to make sure there aren't any big 'squeeze-outs' on the 'wrong side' - those get smoothed with the brush.
If I were using a wood bulkhead, I wouldn't bother coating the edge, as it will be buried in the fillet anyway. I do make sure to sand a 1" strip around the bulkhead edge to help bond to the fillet. I also make sure to wash and sand the hull inside to get a clean bonding surface.

Forward bulkheads used for foot boards sometimes get glass bias strips on the forward side, even though I 'know' that the bulkhead can't move forward because of the taper of the boat.