It's not for any particular weight or portion. Sticking with the macaroni cheese example, if a Sainsburys mac n cheese was 380 cals that would equate as 19 syns, whereas if Asdas was 460 cals that would be 23 syns. The problem with using this rule is it doesn't take account of the foods that are free on SW. On EE and green plans pasta is a free food so the syns for a mac n cheese would actually be lower than the result you get from the 20 cal rule.
Looking at the nutritional info on the pack I know that a 400g Asda mac n cheese actually contains 432 calories, which would be 21.5 syns by the 20 calorie rule. If I look it up on the SW web site though, I can see it's really only 17 syns, because SW have taken off some syns because the pasta in it is free. This is why many people prefer to cook themselves, it so easy to make the same thing yourself with fewer or no syns at all (check the recipes sections on here, they're full of great meals that are low in syns). Or to shop smarter - if I had to go for a shop-bought ready meal I wouldn't waste all those syns on buying the mac and cheese, I'd go for (say) a Tesco ham hock n potato gratin, which is 5.5 syns for the whole thing, or a can of Sainsburys Beef ravioli which is completely syn free!