Hello Michael, probably the best starting point for you is to obtain a copy of the book Practical Junk Rig which will outline all the various considerations when converting to junk rig. This includes placement of the mast. Another way to go would be to obtain a line drawing of the vessel which shows both the bermudan sail plan, and underwater profile including keel or centerboard, and then some of our knowledgeable and experienced JRA experts will be able to help you with this. But it is all dependent on establishing the Center of Lateral Resistance of the hull, and then determining where the Center of Effort of the sail plan should be.