I generally copy what I consider to be 'better' commercial designs for things like coamings- brands like Wilderness (WS) , NDK, Mariner, etc..
I don't think I've seen many commercial coamings with less than 1" of width under the lip (so I'd add that 1" to your riser width).

BTW, IME, the thicker, more rounded lip edge of a wood coaming doesn't 'grip' the sprayskirt as well as a thin glass lip. Some glass coamings also curve 'down' which also helps grab the skirt.

The larger the cockpit opening and the more 'straight' sections in the coaming, the tougher it is to keep a sprayskirt attached.