With an arm, you normally get the movement that it came with. You adjust the throw by deciding where the hole for the bit of string goes - the further out, the greater the throw but the less the torque, the further in, the less the throw, but torque increases. And, you can double the line back to a fixed point to double the throw, same rules for torque.
Ideally, when the sails are as fully pulled in as you want them, the arm should be facing as directly as possible away from the line of the string - where it is when fully out is relatively unimportant. If it can go just over centre, even better, as you then lock the servo so it does practically no work in this state, saving on battery use. The object here is that you usually want the most pull when close hauling, and with this arrangement, the servo uses more of its travel for a smaller change in line length, multiplying the pull against the sail when its wanted, rather than the other way round.