It's actually kind of interesting, they used wax tablets as scratchpads back then. This practice actually continued well into the early modern era because paper, parchment, and vellum were quite expensive and people wanted something that could easily be erased. So most likely they would have written the calculations in wax and only put the final results in clay. By the way, cuneiform tablets are essentially pottery, not rocks. They were basically clay versions of the wax tablets I linked that got fired in kilns for preservation.

If you're not dual-rocking you're just wasting efficiency

If you write things down instead of just remembering them, you'll lose your ability to remember anything at all and will eventually start forgetting what you want to write by the time your chisel touches the stone!

Plus think of all the wasted rocks whenever you make a mistake and need to start over! What if we run out of rocks?? What will we stand on then?

Yes and you need to sign into that rock using Authenticator or it won't let you put it down unerased.

On the sand/ground.

In your own mind? I mean, it shouldn't even be a question.