If you don’t hit 500 calories exactly it won’t make you stall. 500 calories is a somewhat arbitrary number, the idea being to eat approximately 1/4 of one’s normal caloric intake, but there is no special fasting magic that happens at that level. What’s better will depend on you: some people do better if they stick strictly to the limits, others find 500 calories too restrictive sometimes; if you eat 600 calories on a fasting day, you can still lose weight. Much better to stop at 600 than to think, “oh, I’ve blown it, I might as well give up for today.” The important thing is to be honest with yourself about whatever you decide to do; weighing or measuring your food and counting those calories on fasting days can help you get a grip on how much you’re eating on non-fasting days, too.