Ben, the traditional answer about leeway is that it''s just experience, since various designs of boat may have different rates of "sideslip" or leeway. It varies too depending on what point of sail you''re on (more leeway sailing close-hauled than on a broad reach, for example), wind strength, height and type of seas, and how much you''re heeling. On a passage of some length, you might head, say, 5 degrees above the charted course and see how it goes.
The "modern" answer, if you have GPS
, might be to compare your GPS
course-made-good to your course steered. The difference (assuming there''s no current) would be your boat''s leeway.
As to leaning sideway ("heeling") most boats are designed to heel about 10-15 degrees for maximum speed sailing upwind. At 20-25 degees, it''s time to reduce sail. 40 degrees is extreme, any sailboat heeling that far is carrying way too much sail and is overpowered.
Excessive heeling also creates excessive leeway, as the hull and keel become less efficient at resisting sideslip when they become less "vertical" during excessive heeling.