360/21 without stackup (math question)

I want to set up a rotary table on my mill. How do I get the table to travel (for instance) 1/21 of a rotation repeatedly without tolerance stackup? I would rather have "jitter" in my drilling than accumulative error. These numbers are just an example; I am not necessarily concerned with these specific numbers, but rather, the concept.

Using these numbers, 360/21 is 17.143...., but when you multiply that by 21, it=360.003. How do I get it to end up "exactly" on 360? I know if this was "regular" programming, I would calculate it like this: 360*x/21.

Is this correct, assuming I'm using variable #1?
If so, how many decimals will it be accurate to? Will it align precisely on 120, 240 and 360?

Thanks!