It’s not strictly necessary for a small plate like mine but surface plates are usually balanced on either Bessel or Airy points. Both serve to provide the best possible support against gravity. Surface plates have one important characteristic — their flatness. To maintain that precision flatness they must be balanced on points that will minimize the deflection caused by gravity. Even 100mm thick granite blocks will suffer from some sag over time due to gravity.
It might be barely noticeable without high-precision equipment but it will be measurable. Airy points ensure that there will be no slope on the ends and Bessel points ensure that a beam will not change in length. That will also minimize any distortion to the surface, so they are ideal for surface plates, although both types are used. Bessel points are calculated by multiplying the length of the block by .557 and this number is subtracted from the length and the result divided by 2 to arrive at the location of the points from
I haven’t used the level since I first set up this lathe a couple of years ago
Boring the holes for the Bessel points that will support the block
Welding the angle iron to contain the granite block
Tapping the Bessel point holes