the Rotovecta sensor isn't very accurate for wind direction below 5 knots. The type with an arrow in addition to the cups (as they use in the ST60) is much more sensitive at low speeds.

Funny thing is, where I sail the charts are very accurate on the whole. Unless it's very gusty, what I often do is set the autopilot to track the wind angle, then get the sails "dialed in" and then just keep watch -- and tweak occasionally. Yes I'm lazy. ;0

PS -- the ST40 display head can be a slave remote unit on an ST60 network for a fraction of the price.