It would also fit something like a set of concentric circles about 9 foot across. Just like you might find on a big millstone.

Thanks again - I didn't know how big the original stone had been. I'd just assumed something without thinking about it much. But I'd say it was roughly two and a half megalithic yards across. Thom was into that measure. He emphasised that the measurement point, of the grooves, was at the centre, or middle, of them. I should take a metric ruler next time, for balance ...