Hi Colin,
I think much of the error you are concerned about can be eliminated with proper testing technique. First repeat each test several times.
Take the same measurements on both port and starboard tacks.
When setting/approaching a position for taking a data point, approach the point from both sides and take data on point. That is sail low and rotate the boat clockwise to test point and then sail high and rotate the boat counter- clockwise to test point. I think this will wash out most of the errors you are concerned about.
Good Testing,
Bill