Skimmed milk has to be between 0.1 - 0.3 % fat to count as skimmed and even then you can only have 350ml - it's never free. Of course the powdered stuff will be full of calories when it's dried, but by the time you add all the water needed it'll be loads less!
I just calculated the syns from the nutrition on
this one:
You use 57g per pint (568ml) which works out at 10.5 syns per pint = 1.85 syns per 100ml = 6.5 syns per 350ml (i.e. 1HEXA)
Normal skimmed milk works out as 2 syns per 100ml = 6 syns per 350ml (i.e. 1 HEXA)
So there is pretty much zero difference once you add the water. Powdered milk is just the normal stuff dehydrated. As long as they dont add anything else you're fine.