I Tony, I remember reading somewhere that a good rule of thumb was if you had 25mm elevator deflection measured at the trailing edge for a "normal"stab, then to set up your fully flying stab with the same amount of deflection measured at the trailing edge.

I can't remember where I read it and it might have been complete hogwash, but it stuck in my head for some reason.

Pattern planes with fully flying stabs are like hens teeth, but the glider folk tend to have more experience with them. My RES glider has a fully flying stab.