Level, like an Egyptian. This method is both cheap and very accurate. Note the use of windshield wiper fluid for color & anti-freeze.
Unfortunately I couldn’t come up with a way of doing right angles I trusted, so I ended up getting a laser level; which is a pretty awesome toy and confirmed how accurate liquid leveling was on top of giving me a perfect 90°.
A perfect right angle if ever I’ve seen one.
End of the day, with every mason strings finely adjusted to a perfect & level square. It doesn’t look like much but it took 7 hours to get there.
Unlike the pricy electric laser level, the liquid one can sleep outside.