Stitch Shift in Spiral Crochet

Auntie

Hello,

I was wondering if there's a rule for evaluating stitch shift when crocheting in a spiral…

I always have a little trouble visualizing the right place to align my increases and decreases in such case.
I've empirically estimated that I have a one-stitch shift every 2.5 rows…
It's not a very practical formula when there are a small number of rows, but I try to work with it ;)

Do you think this is true, regardless of the yarn you use?
Do you know if there is a real rule?
Could you share your own experience?

Thank you in advance for your answers.