Oh! You can add 15ml (1tbsp) of white vinegar or lemon juice to enough milk to make 1c (240ml) total volume (i.e. add the spoonful first and then fill to the line)... do the math if you need less or more; that'll mean you won't waste any. (Please to note that bottled lemon juice often contains preservatives that give sensitive people issue; vinegar is its own preservative...

Flip side of this, cheap buttermilk *also* uses this method, so don't use it to try to make gravy as well as biscuits with store-bought buttermilk; the result has rather a lot of pucker factor... found that out the hard way MANY years ago.