I've done p90x three times in the past... When I followed the nutrition plan the first two times (did something different the 3rd) I just used a cup measurement. It didn't really specify that, but thats what I followed both times and had good results.
It's a great program for beginners who are looking for some structure and get use to various basic body weight and free weight exercises. However, just keep in mind that you're looking at losing weight more than building muscle mass. Don't get me wrong, you'll gain/tone some muscle weight, but p90x's main goal is weight loss.
Good luck, have fun with it!