Tesco white sauce mix

Used a Tesco white sauce mix to make lasagne for tea.
Confused bout synning it (sorry if obv but don't normally use packet mixes):
Its a 25g packet
but it gives the following 'when made up with semi-skimmed milk' but i used my HEX A of skimmed

100g
70Kcal
4.4g protein
7.7g carb
2.2g fat

1/4 pkt (80g sauce)
55kcal
3.5g protein
6.2g carb
1.8g fat

As i used my HEX for the milk am i right in using the 100g amounts? I used half of the sauce so 35 cals = 2 syns

Anyone any ideas???

I'm fairly new to all this so I wouldn't take my word, but sounds right to me. That's how I'd work it out anyway