I wouldn't recommend the "eyeballing it" due to the fact that the rudder geometry changes when the blades are lowered and locked down.
Not if the rudder pintles are in-line with the hull axis...this is actually how I align the rudders on the 20 because it is practically impossible to determine "horizontal" to put a measurement line on them. Horizontal would be the line that the water flows when the boat is sailing...the bottom has rocker and the decks sloped...what horizontal line do you pick?
My a-cat however, has canted hulls but apparently the rudders are mounted on a slightly different axis - they toe WAY out when raised compared to down.
