Are you using leech tell tales? The best way to know if you are over sheeting is to watch the tell tales on the leech (each one attached at the edge of the material). As long as they are flowing (indicating attached air to the leech) then you have not over sheeted. Once you do, the leech tells will cease flowing as the sail loses attached airflow and will swirl behind the sail or just droop. There should be at least two tell tales on the leech - one in the bottom third to indicate traveler trim and one in the top third to indicate mainsheet trim. Regarding trim in light air, follow the old saying, "When in doubt, let it out". Once the sail is eased and you sail a bit lower, gently trim in and adjust course accordingly until the leech gets too tight according to the tell tales and then ease back out a bit to get the flow going again. This applies downwind as well as upwind. In light air it is more likely that leech twist will need to be increased as there is likely to be more wind shear, especially the lumpier the water gets. More twist also gives the sail a wider groove but if the water is flat and the wind is pretty steady then sheet in as long as the leech tale is flying since a narrow grove is all you will need and you will be able to sail as high (or fast) as possible with the breeze you have.