[makman94]

if we have a ship with a mast of 26m at 3000m and your rec writes that mast=25 then you will measure a distance of 2885.so you will have a false of 115m at 3000m .if it is closer the false also getting smaller (at 2000m the false is 77m )
so, LGN1 is right that a 5% can be 'accepted' if you don't want to use the b+d mod.
now,trying to eliminate this false by eye (aiming to a little lower spot from the one showen in the rec) its always based on your own abillities and experience in game.its the same with not calm seas ,always its up to your abillity and experience to find the waterline!(this false will be always there to distinguish the bad and real good captains)

edited