The linear drive should have an adjustment screw on the arm so that it can be adjusted to show when the rudders are central. you will need to centre upm the rudders then loosen the screw and adjust until the guage shows the centre position. May take some fiddling to get right cause sometimes when you centre it the guage won't read correctly in the port or starbord directions (won't go all the way over). May need to re-position the whole unit.