Hello Gentelmans,
Some older lathes doesn't have an angle scale on a toolpost. It is possible to adjust the toolpost via laser pointer with or without colimating lens and some linear scale on e.g. spindle.
Here's my document with calculation of residual systematic error. Suprisingly it is possible without much effort to get 1/6 deg accuracy. I couldn't find any error in calculus.
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Comments and corrections appreciated --Jack.