You don't need to know any formulas ( "formulae" ?), fancy equations...or estoeric geometric princinples. Just use the loupe on the ground glass.

1. Determine the (approx) plane of focus. This is the most critical part and requires good judgement. This could be a horizontal plane, a vertical plane or something in between , in which case it would require both T & S.

2 a ) On the furthest part of the plane ( furthest from you that is ) tilt(or swing) in very small amounts till its in sharp focus.

2 b) Focus (by turning the knob ) on the nearest part of the plane till it comes into focus.

2b) will cause 2a) to go slightly out of focus. Repeat 2a ) and 2b) a couple of times (or more) and you are done.

Don't forget to stop down for DOF on either side of the plane of focus.