I also have a simple suggestion for the white underside, if you allow me. Take a block of wood of the right hight and place a pencil on it horizontally. Place the model on a firm stand and draw the waterline around. Usually the lowest part of the flat tuck is a little bit below the waterline, just like the broad part of the rudder and on the front side you can start close above the protruding part of the stem. Mostly these ships lay a little bit deeper at the stern, so the keel was not exactly parallel to the waterline.
I hope I was clear.
Good luck
Ab