Unlike the C-22,(unless you have a Rudder-Craft rudder), the C-25 has a balanced rudder, so the distance from the pivot point to the pin isn't that critical. The further away, the more leverage but less travel, the closer to the pivot point the more travel, but requires more force to push the tiller. The ST-2000 shouldn't have any trouble handling the tiller on the C-25. I kind of went through the same situation mounting the ST-2000 on our C-22 MK-II and choose the position slightly forward of the recommended distance to the tiller mount. The critical measurement is ensuring at the exact center position of the autopilot is at the exact center position of the tiller, and it's at 90 degrees from the center line of the boat. Yes I lost a slight amount of tiller travel, but it's not been an issue with hundreds of miles logged under the steady course of the autopilot. The pin needs to be straight up and down, as well as the Autohelm mount on the combing,(or where ever it's mounter). I didn't want to install any brackets on the tiller because then I would've had to have a custom tiller cover made.
Absolutely LOVE our Raymarine ST2000. Couple it up with a wireless remote and you have the ultimate steering machine!
Don