there will be no free food allowance in the packet, so you can look at the calories and use the 1 syn for every 20 calories.
then if it has to be made up with milk you can either use the milk as a HEa or you will need to syn it. Again the milk has no free food allowance, so you can work out using 1 syn for every 20 calories.
You may find that the nutritional information on the white sauce packet includes milk. Some do and some don't.