this is really interesting, and I thought there would be a difference if the calories came more from fat than carbs or protein or vise versa, so I went onto manual syns online and put in three different versions of 100 cals, firstly with 10g of fat (9 cals per gram of fat) and no carbs or protein...= 5 syns.. then put in 5g of carb, 5g of protein and 6g of fat (4cals per gram of carbs, 3.6cals per gram of protein) it also came as 5 syns! so then i put in 10g protein, 10g of carbs and 2g of fat... also 5 syns!
I was convinced in my head that it should be different, but it was always the same. so I have to agree with jaylou, as a rule of thumb - 50 cals is roughly 2.5 syns. if you can add it up its better to double check, but as a rule of thumb its a good figure to go by.
I am actually amazed to be honest, but there you go.