Pull the main in just enough to go as fast as you are comfortable with. Best to start with the traveler out about 6 - 12 inches from center then sheet in more until you get use to the power / speed then you can move the traveler in towards the center more and let the main out until you get the feel for the boat. As you are learning it is best to not cleat the main sail in and keep the sail in your hands so you can release it quickly when needed. In no time you will be sailing with the traveler near the center and sail in tight! 18 -20 is pretty windy for learning but it will make you lean fast.