Cook mashed potatoes in buttermilk
I would advise against it. Cooking in acid makes vegetables firm. Sometimes this is a good thing, but if you want to mash your potatoes afterwards, you want them soft. Else you'll get the wrong texture. 

There are numerous recipes for adding dairy products to the mashed potatoes after they have been cooked and mashed, and they taste well. You can look for them, or experiment yourself. It's a matter of what taste/texture you like, there aren't any physics involved which you'd throw off with a "wrong" ratio.