If the servo is set up so that 100% of the stick movement coincides with the total desired movement of the controlled surface you will be able to make a fine adjustment to the end point. The DX6i has the facility for exponential which can soften the immediate response of the servo - useful to prevent overshooting when making turns, Try 30% on your rudder. On the left stick you can use the throttle curve to make the stick less sensitive as the sails are pulled in. This makes it easier to control the last 10 degrees of movement of the sails to finely tune them when heading into the wind. I would add I know little about sailing as a newcomer but I've been flying rc planes since before 2.4ghz came out. I suggest you go to the local rc flying field with your gear - there will be someone there who can show you all the features of the DX6i.
Cheers Ian