John,
Ref your experience in the lakes.
Was their any preferred system you employed for 'trimming'? Some bulk carriers had trimming lights (Green, White and Red) atop the wheelhouse. Others ( I believe ) used the spirit level on the Magnetic Compass...Yeah right!.
I used to rig up a clear plastic tube pipe amidships which extended up graduated supports at the ships rail (about 2m high). The pipe was filled with water and 'Hey presto'. Ideal subs for reading the offshore midships draught mark when there was a big sea.
Bill