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Stones like this one come from factory with a weird spiky surface, and pretty much cut like a super coarse diamond plate. And are messing both your grit estimation and speed. After a few knives, it...

You can find a piece of iron/steel, grind it, and see how deep the scratches are. I find this to be the most reliable method, as long as your stone is lapped. After that, it's experience.
Or you...