Hi banana21, live by the adage “Use it or loose it”. The targets are 600 and 500 cals per Fast Day. If you are a little under or over it does not really matter. Other posters have also questioned if they can work off say 300 cals on a fast day and therefore eat an extra 300 cals to add to their fast day calorie target. The answer is strictly speaking no. Do this to suit yourself, the idea, devised by Michael is to stick to the target on your prefered days in order to achieve the best inner health targets. By all means experiment until you find what suits you this is a very individualistic way of life with few guidelines.
Good luck.