> of course if you have a known flat surface you can save effort by making the new plate flate to the known flat.

All the effort!

one of the good things about granite flat references, is that they last for ages, so you can get them reasonably cheaply second hand, if you can find a second hand machine shop specialist.

Even if your second hand plate is no longer flat enough it is still close enough so there is very little effort needed to bring it back to flat. (machine shops often check them on a regular basis and bring them back to flat as needed)