I don't want the waterline.  I want to level the boat on the trailer.  I'll explain.  I want to add a "moveable" gunwale-mount boarding ladder.  I want to be able to hang the ladder aft, not off the transom, but off the gunwale, to starboard.  I also want to be able to hang the same ladder forward, again, off the gunwale to starboard.  If I install mounting tabs flush with the deck, my ladder will hang askew due to the boat's shear.  So I need to install pads that will level the mounting tabs in relation to the way the boat will sit in the water and not in line with the shear.  I need to level the boat on the trailer close to the way it will sit in the water, so that I can get the mounting-tabs level to the way the boat will sit in the water.