This post inspired me to create a 3D printable tool to help set the 12 o’clock postion. Here it is on Thingiverse: https://www.thingiverse.com/thing:3361094
(and a picture)
It fits on top of the sprocket and has a space for a bubble level to be glued into it. It also places the bubble level 3 inches away from (the center of) the sprocket for increased accuracy of the level.
I also wanted to mention that you want to set the 12 o’clock position in relation to the top beam (so the the top tooth of the sprocket is exactly perpendicular to the top beam). So if your top beam isn’t perfectly level, then that could also be a source of inaccuracy when using a bubble level to set your 12 o’clock position.