That doesn't work for all foods though - if the product has free foods among the main ingredients it gets a free food allowance and you can't work out the syns using this formula. There are also some foods that don't qualify for a free food allowance even though they have free foods in them eg pre-prepared soups, sandwiches and pasta sauces.
the 1 syn for 20 calories, works for all foods that do not qualify for a free food allowance.
things like cakes, biscuits, chocolate, icecream, ready made soups/sauces, crisps, drink, pizza, pastry items, ready made sandwichs etc
all other foods you need to use the online official slimming world syn calculator, which applies a free food allowance so works out as less syns. Things like ready meals where the main ingredient is a free food (for instance chicken), savoury rices (where the main ingredient is rice). etc