$1936

I have a 64 slot disk, and the pattern I want to make is 40 strings. When I’m setting up on the disk how do I do the math so the strings are all evenly spaced?
One thing I tried was setting up the opposite sides first (12:00 then 6:00, 3:00 then 9:00), then trying to space the strings evenly between but that didn’t work.

It doesn’t have to be exactly evenly. What you’re doing sounds fine.

Is there a way to figure out the math though?

Sure, but for this one you can't set it up evenly.

You have 24 empty slots (64-40)

You have 20 pairs (40:2) which means you also have 20 gaps between the pairs.

Each gap is approximately 1,2 slots wide (24:20).

You can achieve that by having 16 gaps that are 1 slot wide and 4 gaps that are 2 slots wide (16x1+4x2=16+8=24). That means to make it as even as possible every 5th gap is 2 slots wide and all the other gaps are 1 slot wide.

There are also other solutions to this. You could for example also have all gaps be 1 slot wide and only one being 5 slots wide (19x1+1x5=19+5=24).