When I look at a brush holder from above I can see that the brushes are set at different angles. I assume this has to do with the diameter of the commutator, degrees between slots, number of slots and brush width?

Is there a formula that calculates the optimum brush position using these parameters?

I am looking at a brush holder on a DC PM with four brushes and a commutator with 29 slots (starter motor) I can see the offset is 20 degrees at the commutator face but wonder how this was arrived at?