Hi TT,
I understand your fears, but consider - with the rudder on, the CLR will move backwards. Adding the fin will move it forwards. I wouldn't, at this stage, personally worry about quite where it's going to land, now you're in the right sort of area, which I think you clearly are. Here's why: your fin is as much to carry ballast low down, in order to provide a greater righting moment, as it is to increase lateral resistance, which will already be considerable from the hull form, what with its full-length keel. So IF (big if) you end up with copious helm of the weather or lee variety (and I really don't think you will) you could always decrease or increase the fin area to correct this.
Regards,
Andy