I am thinking about scratch building a twin engine electric more like a seamaster not like a cub (what do you call each?) What are the parameters for hull width, length, angles etc.

Most of what you need is here:
http://www.smilesandwags.com/Floatsite/westwood.html

Although the article is difficult to read, the information will be useful. I have on question though. What is "dead rise"?

Follow these to the letter, never fails.

Antique's attachment is a reference to Chuck Cunningham's landmark article which is the rosetta stone for float-flying. It is applicable to floats, but the article I sent has information pertinent to flying boats. ...Only one float-the fuselage.

Dead Rise , as I understand,is the angle the hull adopts when not making any lift. V-bottom hulls have deadrise, flat-bottom hulls don't, but they skip when they land.

Jim,
I used the formula for a single float, the type that looks like a torpedo hanging under the fuse. The beam would be rather narrow, but the equation IS for a float not a hull. Could I widen it to what a normal fuse width would be?
The single float also has a larger portion of the float ahead of the step/CG than does the twin float arrangement. What would the ratio be for a hull design?