When I started my coleslaw this morning, I realized that I had less than half the amount of buttermilk that I needed. Our local store changed milk distributors, so it only carries a 5% saturated fat buttermilk, which it calls gourmet. I also cannot get a good plain yogurt there. However, I have a partial tub of Greek yogurt, so I googled to find out if I could substitute, and I found this article by P. J. Hamel:
https://www.kingarthurbaking.com/blog/2019/01/28/how-to-substitute-for-buttermilk#:~:text=It%27s%20also%20an%20easy%20ratio,2%20cups%20of%20buttermilk%20substitute.
As I was using the substitute for a buttermilk dressing, I needed it to taste like buttermilk, and when P. J. gave the taste award to the combination of 1 part Greek yogurt to 2 parts 1% milk, I followed that formula. The dressing for the coleslaw turned out very well. I can taste a bit of a difference, and I did have almost half of the buttermilk, but it kept me from having to go to the store. I did the 1:2 by weight, so I do not know if it works by volume. I have some leftover, which I will use in baking.