don't forget you can have milk as a HEa
350ml of skimmed as a HEa or 250ml of semi skimmed.
If you are not using it as a healthy extra, then milk doesnt qualify for a free food allowance so you can work out by using the 1 syn to every 20 calories, by the nutritional information give on the bottle or carton.
Of course you will have to measure exactly, as like silvermaneuk has rightly pointed out. A mug size can vary greatly.