I did the line with the help of a laser beam. I heeled the boat 22.5°. That is easy on a small boat especially if you have 6 chain hoists above.
Finding sheer with the help of an inclined plane is nothing new. I guess its been don for thousands of years with the help of a string and a stick on the stem and stern. Just angle the sticks to the desired degree. This has the advantage that on a bigger boat you do not have to heel her.
At the same time I did a sheered waterline.