That formula gives the number of units to send to maximise returns. It is used by the land traders on Alliance and I have accepted it on authority, I have not myself gone to the source materials. However I have used it and it does seem to work. The relevance comes very early in a set when you are hitting a country with little or no defence. By the time countries are decently developed the number of units required to break will always exceed the number generated by the formula. It is of continuing use if you are taking a hit on landfarm.

If you want to look into it the expanded formula looks like this,

0.068*(0.43)*(1.5)*1.13*targetland*myland/20

so that might give a clue as to how each of the components has been derived.

(Matching networths is much more significant than units sent.)