I meant risers to get the rear beam further from the water. Have you noticed how much the rear beam slaps the water and slows you down when running downwind?
anyway...
take the dpn of the original boat and then use all the multipliers to get to a decent starting point. There are modifiers for oversized main, no jib, jib on a boat normally without, etc etc etc...
http://www.ussailing.org/portsmouth/tables06/modifications06.asp
tell us what you did to change the thing and we can then calculate a starting point at least.