But buttermimlk is aready nasty skim milk anyway. If you don't want to buy a quart or a pint if you can get it then buy the powdered stuff from Sacco made for baking. Don't reconstitute, just add the powder to the dry stuff and then add your liquids.
To equal 1 C. buttermilk use 4 TBS Sacco with the dry ingredients Add 1 C. water to the wet. 2.5 TBs to the dry...add 2/3 C. water to teh wet ingreds. Keep the Sacco in your refrigerator after opening.