The tables are fairly clear if you read the class weight table.

First of all, a boat sailed sloop must use the sloop number, not some sea-lawyer trick of uni plus jib. So call it sloop and start with the 77.0 number. The class weight tables offers both Super Cat 15 "Single " or "Double". Whatever that means is irrelevant since the footnote says, "SuperCat class rules state that in open class racing the number of persons on board may be increased or decreased." That means if you sail solo and you weigh more than 150 your good at 77.0. If you sail with crew and weigh at least 260 your good at 77.0.

Our fleet has a well-sailed SC15 sailed solo sloop and he uses the 77.0 base number and he kicks a$$ with a custom Bobby Johnson squaretop (requires an MN mod of 0.995 giving 76.6). Go Tommy!