Chisanbop or chisenbop (from Korean chi (ji) finger + sanpŏp (sanbeop) calculation 지산법/指算法), sometimes called Fingermath, is a finger counting method used to perform basic mathematical operations.

https://en.wikipedia.org/wiki/Chisanbop

You might be already doing this. If you strum your fingers of your right hand by pressing your index, middle, ring, and pinky to your desktop, and then do the same thing again starting with your thumb, you've just counted from 0 to 9. Do the same on your left hand and you've gone from 00 to 90. It's really easy to do simple math this way by counting on your fingers.

For stimming purposes, you might just start by counting up or counting down, then maybe counting up by twos or counting down by threes.

This is the approach that I've known for many decades now. I've seen YouTube videos of kids doing amazing fast calculations like multiplying large numbers using what looks like a different method in that their hands are in the air. I'll leave it to you to Google the other approaches if this direction interests you.