No you do not need to syn the milk if you are not having the full amount. There is nothing in the SW literature to say this. All it says is that you may not get the full recommended about of calcium. But if you are eating yoghurts etc then this should not be an issue.
And NO, you are not allowed to carry HEs over from one day to the next - they are use it or lose it - so no using the leftover milk the next day.