Mactec, can you elaborate on the "magic number" for a single under-table ballscrew? Seems like it would be more of a "magic ratio" between spacing of the two carriages on each rail and the width between each rail. Obviously as peteeng says 1:1 is ideal, square arrangement. However, it must scale. So is 1:2 acceptable? say, 250mm carriage spacing and 500mm between rails? Assuming medium preload a la hiwin ZA.

I'm guessing that since the spacing of carriages along the rail of typical routers is around 200mm/8", you are saying that given that, 500mm would be the largest acceptable rail spacing. Which is 1:2.5 ratio