I don't know why it wouldn't be valuable to roll balls on the bare slate. It will pick up any tiny steps at the slate joints and any slight roll off that would be masked by cloth. Of course fancy levels are the gold standard but they eventually drift out of calibration and are only able to tell the story of the area they are placed.
I rolled balls for hours when I did my own table because I didn't have multiple known levels. It took me a long time but so what, I do one table every 5 years. I built a little "stimpmeter" type ramp to roll without spin...not as fancy but similar to Dr. Dave's shown below.
Again, if you have a bunch of precision levels...great. If not or if you just want a double check then what could possibly be more definite than slow rolling balls?
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