[contrary1]

My substitute for buttermilk is not an exact science. None of my cooking is.
I keep a container in the fridge of powdered milk already mixed up. I don't measure this either anymore. Just some powder & milk enough to make it look like 2% milk. More or less water.............it's still milk. Then, I pour out the required amount for my recipe, minus a bit, then add a glug of vinegar. Just enough to sour the milk. Stir a bit & let sit while I measure out other things............then, it is ready.

I find that having the powdered milk already mixed up in the fridge, I use it. Now, since I absolutely don't buy regular milk, it isn't a problem. But, before, I would use out of the regular gallon rather than mix up a cup or whatever the recipe called for.