Ciao Martin,
Sorry but... I can't help you about the "total lateral resistance". I don't use this point in mine projects becouse from my point of view it is not so effective and give not so good "results" expecially in little models.
I suggest you to utilize the center of keel (deriva) and the same parameters to place the center of the sails on your Fat Boy Slim and verify if it work quite good.
I suppose that could be ok.
Let me know the results
Ciao