Router guide bush calculations

[Baldhead]

Somewhere on the internet, I watched a video where you were shown on a white board, how to calculate the offset using different size cutters and guide bushes, anyone else seen this and could they tell me where it is.

Baldhead

 

[twothumbs]

Dia of guide - dia of cutter, halfed. ie 16guide - 10 cutter = 6. 6/2 = 3. So 3mm bigger or smaller, depanding on what it is....inside or outside.