It is accurate for anything WITHOUT a free food allowance - which covers, sweets, snacks, crisps, biscuits, cakes, icecream, sauces, tinned soup, pizza, pastry covered items etc.
If your food item however qualifies for a free food allowance, then you must use the syns calculator.
So if someone is asking for the syns of a bar of chocolate for instance, or the syns in a jar of sauce/tin of soup etc then it is right to say it will be 1 syn for every 20 calories, as non of those items qualify for a free food allowance.
If however someone asked for the syns in lamb shanks with gravy or a savoury rice or a fat free flavoured yoghurt etc. Then these would need to be checked on syns online or the syns calculator, as they do have a free food allowance.