yep it is pretty accurate. I haven't found anything where it is wrong, and is the same way the snackulator calculator on the slimming world website calculates things with no free food allowance.
of course if something has got a free food allowance, then you need to use the syns calculator.
1% milk is not skimmed by the way. Skimmed milk is defined as containing between 0.1% and 0.3% fat so the stuff you are asking about is just standard skimmed milk and would count as such.