That sounds like a very interesting technique. Instead of using volumes, why not use weights and the metric system? Electronic scales are inexpensive. The most common cooking scale has a readability of 1 gram (g), but you can also obtain them with readabilities of 1/10 g, 1/100 g, and even 1/1000 g. It's useful to know the weights of your dishes in advance, but not absolutely essential. You would also need a calculator, unless you have your spreadsheet within easy reach.
In thinking about this, you could weigh out any food, then weigh what they didn't finish, subtracting the latter from the former, and you'd know the weight of food they actually consumed. I guess it would be a little complicated when they would eat wet food and dry food, because there'd also be a ratio between those two. I wonder if this technique would be more trouble than it's worth? It would be another set of data points to track. Cats tend to be creatures of habit.