Just thinking this through.........If he is building / adding on to the boat, he wants everything square and plumb to match the boat, not necessarily to match how the boat sits in the water as that changes with weight distribution. I would adjust the stands so that any bulkheads that run thwart ship are plumb as the boat sits on the ground. This will indicate it being level fore and aft Most boat haulers will be sure that the boat is level as it sits side to side. A simple level on the deck or transom would indicate level from side to side. Thoughts are welcome correct me if I am wrong.Set a laser to level and adjust the waterline to match.