I think you will get far with the A-class LYS number in one-up mode and the F-18 number (1.67) in two up mode.

But, the only way to get a LYS number is to approach the LYS committee in segelforbundet, or perhaps better, the multihull LYS committee at: http://www.multihulls.org/lys2007.html
Expect the number to be quite hight to begin with, as they prefer to err on the high side.