To get other increments, use 1/6 and 1/8 together. That is, add 1/6 and subtract 1/8. That is the same as adding 1/24.
For the map you posted, the proportions definitely seem a little different or offset from other areas on the whole map. The tool was not designed to handle that. It assumes a fixed grid has been used. But that grid can have different vertical and horizontal sizes and the sizes do not have to be whole pixel units.
For the map you posted, I would use 20.458 (20 then add 1/6, 1/6, 1/8) and vertical of 20.333. That is a off on various portions of the map. But since FG allows you to move the grid a pixel at a time, you could move the grid a little bit whenever it gets too far off.