It works on everything that doesnt have a free food allowance, so all sweets, crisps, cereal bars, bread etc (although of course some of those can be taken as HEX). It wont work on anything that has free food ingredients (so you couldnt work out how many syns in a sausage for example on orginal or EE as the meat would have a free food allowance).
The 1 syn = 20 cals rule is a god send when shopping and deciding which biscuits or treats to buy. Or what cake I can get away with eating when someone brings birthday goodies into work (as long as the calories are listed of course)
I've said it before and I'll say it again - I do think that 1 syn = 20 cals should be a sticky topic - there are so many syn queries that could be answered in one fell swoop (assuming people read the stickies of course!)