Never heard of Fast 800 - sounds interesting - random youtube video https://www.youtube.com/watch?v=wDEEal3nO9E

Sounds like you vary the caloric intake back up on eating days

From the video it doesn't appear to be particularly what we'd call "Low Carb" around here but he mentioned the veg and fruit to select "lower" carb versions. If it's better that you where eating 13 weeks ago, what have you got to loose?

Go For it! See how it works for you