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Can't comment on leeway for an M and it depends on a lot of variables, point of sale, sail trim, weight distribution, weather. Best to just get out and observe your wake to get a feel for how much leeway you are making on the day.

Macs shouldn't produce much deviation as standard as there isn't much ferrous metal I can think of apart from the outboard (but the 'stainless' isn't all completely non magnetic). It will mostly be after market equipment that will mess things up. With a GPS it is a fairly easy exercise to swing your compass and record a deviation chart but I confess I haven't bothered myself. DC electrics can stuff your compass reading very easily. Keep all wiring well away or if it has to pass near (i.e. a compass backlight supply) then use twisted pair cores so they cancel their magnetic field better. Every where else use pairs of wires i.e. +12v and 0v in close proximity. I have seen wiring done in singles that follow different routes and produce large magnetic fields. I know one boat that had a compass that acted as a bilge pump detector!

Dave.