For a while it was on a scrap text document, but eventually I was able to keep the math in my head. It helps to eat some of the same things daily, so I know already roughly what my breakfast, lunch and snacks come out to.

I use the iPhone app called Track and Plan which is also available offline (i.e. if you're on a plane or not "in network" for some reason).
Before this, I used the calorie king book and a small notebook. I record my calories right BEFORE I eat/order/begin meal prep as the case may be.