A lot of the skimmed milks differ but the one i usually get, you will get 125ml of skimmed milk for 1pp which is what i generally use in cereal
semi skimmed is 4pp per half pint. measure and point what you measure then keep it in fridge, then use it freely. i always measure a pint (8pp) and keep it in fridge to use for cups of teas/coffees/cereal ect throughout the day