I actually normally follow the serving guides; I half everything and measure everything. Even the other day I had to get the scales out to measure something like '100g' of frozen chips, because that's the amount it listed on the packet.
You won't starve, you just think you will. But then, I'm the person who uses a calculator in the supermarket to work out calorie contents per day (680 for today if you're so interested.)
Oh, and if people want crisps, they sell some in Marks and Spencers at only 89 calories a packet.