The probable best discussion of barberhauling (or tightening/opening/adjusting the slot distance) can be found in:
http://www.arvelgentry.com/magaz/Achieving_Proper_Balance.pdf
This article was written in ~1974, long before GPS and
Velocity Made Good (VMG) function. A more modern edition would be to use the GPS'
vmg function when testing or preplanning the amount of barberhauling needed. The amount of baberhauling, etc. usually depends on copious previous record keeping from prior trials vs. wind and seastate conditions ... so you can 'quickly' set up the proper amount of slot open distance ...... and while making further corrections in sail shape and trim while keeping ALL the tell tales flying 'perfectly'.
What's missing in the Gentry articles is: There is no such thing as making only ONE adjustment in sail shape and trim --- always at least
two 'adjustments' are required - one to make the basic correction and the second to
return the sail shape back to perfect proper 'flow' of the telltales and was 'disturbed' by making the 'first' adjustment.
The 'usual' set up to
barberhaul, etc. is to 'start' with the barberhauler (or transverse jib car adjustment, if you have one) IN (towards the mast) for minimal slot open distance in 'light' conditions .... and OUT/eased when in 'heavy' conditions. With barberhauling, and above a close reach you 'optimize' at the highest VMG attainable; below a close reach its all a matter of optimizing the postion of the genoa/jib fore/aft position on the rail.
As an aside, the same techniques of using GPS' vmg function are used in setting the proper jib fairlead car fore/aft position when going lower than a close reach and where the barberhauler is of NO value. I list this as virtually NO cruising sailor (and a lot of racing/performance sailors) ever adjusts their jib fairlead fore/aft position when going at less than a close reach or 'downwind'.
