Tests never add errors to a mirror
the laser on the Hubble reflected off the wrong surface (a nulling if test not ronchi), can’t have that on a Ross setup. Ceravolo’s anecdote would be a better ref for setting the distance incorrectly.
the Ross setup for a 14 f4.5 has a tol in setup that’s not hard.
but your missing the point. Using the null is a very fluid, quick and instant view to help judge how to shape the surface. Get the shape correct the rest is just correction, and you can take a longer time with the ke or if or star test to verify.
I have no more to say about the benifits of the Ross null test. Either you can appreciate the process while making a mirror or...
when was the last time you made a mirror using these tests? Seriously.
Errors of the test device add to the wavefront. If you're nulling a mirror/lens against a wavefornt which also has errors of the test device/setup you'll null it but the mirror/lens true figure will not be of the figure yo're shoot for.
If you use a flat in a DPSC but it actually unbeknownst to you contributes 1/5 wave error on the wavefront (due to either the flat being faulty or the setup or both), and you null your mirror, that mirror will in reality be off by 1/4 wave off on the surface -- hardly a "perfect paraboloid".
Sure, using the null is a quick and instant picture of the wavefornt, but unless you can discount the errors of the nulling element from that "null" that null is misleading.
You seem to neglect the fact that the Ross null test is not a double pass (double sensitve) test for the test optic, so a Ronchi Ross test is hardly a good "judge" as you say. But since the Ross lens gets traversed twice it has to be of exceptional optical quality -- because any errors from the lens will be doubled!
Thus, a null is valid only if you absolutely know with certainty that your test device does not contribute greater error than what you're shooting for. If your "Ross" lens is of unknown surface and figure quality, or your setup error is unknown, you can't make an assumption that the null you're observing reflects the correction on the test optic.
As for the last time I made optics using the Ross null test the answer is never. I would never make any optic using a "Ross" lens of unknown quality, and since I don't have a certified test lens, it's pointless. And so is a ":quick look" with a Ross using a lens of unknown quality.
That's why I asked you when was the last time you a mirror using your Ross test and had that mirror tested with an inteferofmeter to see just how good it is? Seriously. To me it makes more sense to monitor the progress using a knife-edge for a "quick look" and/or an IF.