A bit tricky with the measurements, I think. What is really important is that all the steps are the same height. Otherwise, you trip on the stairs. The way you have stacked now, the top and bottom are half as high. Dangerous. Similarly dangerous with small/short stones at the front, they can "roll" when you push off when walking. Remember that the joint is about 1cm.
It would be good if you could guess the number of stones.
One alternative might be to build up the stairs with stones lying down and then cover the steps with tiles. Should work and be cleanable.
I'll draft a bit tomorrow...