It's not really a magic number, the word magic gets peoples attention, not to build a Gantry machine, too big with a single Ballscrew
It does not have anything to do with Bearing spacing if you can't get the ( 2 ) linear rails parallel to each other, ( .0002" to .0005" Parallel ) which most can't, unless the mounting surfaces are machined and both in the same set up 500mm x 500mm and anything in that range can work without to much racking with a single Ballscrew, larger sizes is not going to happen without racking no matter how you space the bearings, unless you can get precise alignment of the rails
What causes the Racking and binding is bad alignment and rigidity, not the bearing spacing, Bearing spacing only spreads the load of the Gantry which is important as well
On my builds I have used from 150mm / 200mm on the small machines depending machine size with a single Ballscrew, and 260mm on the mid size machines with a Ballscrew on each side, this does not prevent racking, only alignment can do that
Here is one I built some time ago that I did a video of this had 160mm but was not a very big machine and in the 500mm window for travels, The Gantry was also a complete box, this can also helps to prevent racking, but again alignment is key to any machine build