Check here: http://www.hobieclass.com/site/hobie/ihca/downloads/hobieu/HobieU.pdf

This should get you in the ballpark.

There is also a section for the TheMightyHobie18 in the back of Rick White's book "Catamaran Sailing for the 90's"

Soft diamonds allow the mast to bend, depowering the main - good for high wind. Hard diamonds do just the opposite - good for light air.