Not tightened fully; on the side of my Prusa, I measured 19.39mm of travel per revolution. I just turned by hand and eyeballed one revolution.
Tiny_Ballscrew_8mm_38_5deg

You can calculate the distance per revolution with the angle, and vis-versa. something like: (Pi x RodDiameter)/Tan(90 - angle) = Travel; is pretty accurate, if not exact. In your case, (pi*8)/Tan(90 - 38,5) ~= 19.991, so you were clearly close.

You can calculate the distance per revolution with the angle, and vis-versa. something like: (Pi x RodDiameter)/Tan(90 - angle) = Travel; is pretty accurate, if not exact. In your case, (pi*8)/Tan(90 - 38,5) ~= 19.991, so you were clearly close.