A-Coaming Lip Width - Anything less than 1/2" is a little too shallow--many production fiberglass boats have as much as 1 1/4" deep recess here under the coaming lip.

B-Coaming Lip Thickness - You're fiberglassing this bad boy, so you could be as thin as 1/8" here. 3/16" or 1/4" would be easier to work with. Anything over 1/4" would be wasted weight. (but easier construction)

C-Coaming Height (below lip) - A lot of boats are about 3/4" here--to give you enough room for the rubber rand or bungie on the perimeter of the skirt to "pull" into this area. Any lower, and you might have a hard time getting the sprayskirt down. Some towing systems use a rope around the perimeter of the coaming, as do stirrup re-entry systems and sea socks, so you would need to have some more space here for one or more of these systems if you plan to use them.

One note: on a couple production boats (Current Designs), the A-dim is about 1" and the C-dim is about 3/4". If you're moving the boat around, it's relatively easy to stick a knuckle in there, if it gets stuck, you could have a nasty sprain.

I think on my boat, I'll shoot for A=5/8", B=3/16", and C=3/4".

Shawn