Laylines are tricky because the tacking angle of the lightning changes very dramatically with variations in wind velocity. Basically - the more wind, the tighter your tacking angle.

In very light air your tacking angle will greater than 90 degrees. In light to medium air your tacking angle will be about 90 degrees. In medium to heavy and heavy air you tacking angle will definitely be less than 90, probably between 80 and 85 degrees.

You can figure this out using your compass very easily for your given condition by sailing upwind on one tack recording the heading (I always use the centerline on the compass, not the lubber lines, because- the math is way easier). Tack and record the heading after you are up to speed. The difference between the two numbers should be close to 90 degrees. I think it is best to try to do this a number of times before the start to make sure that you are not tacking on windshifts. This is also the time to record your high and low headings on both tacks for reference on the upcoming beat.

If the tacking angle is close to 90 degrees, you will probably have a crewmember to leeward, because it will probably be light air. Have that crew sight up the hiking strap (90 degrees) to tell you if you are on the layline.

In practice, since it is not really practical to use a hand bearing compass, I look at the other boats' headings on both tacks while sailing upwind, and visualize my boat paralleling them on the other tack. It sounds hard to do but you get really good at it after a while.

Remember too, you want to avoid getting to the layline too early in most instances. Once you hit the layline, you lose ground with either a lift or a header, so unless it is a persistent shift, or there is a clear advantage (less current, more wind velocity in non-shifting conditions) you should try to avoid the laylines.

Bill Fastiggi SHORE SAILS 1998