Well, I am doing this diet too, and I have heard that it works better if you leave a as long as possbile gap between eating.
At first I had lunch then dinner. Then I tried having breakfast and then dinner. Now I just have dinner and find that works really well for me. I am losing a pound a week and find that I eat really healthy foods on fast days.
Does it make a difference when you eat your 500 cals? For example today I've had no breakfast,(well a coffee) then 100 cals for lunch, and about 400 cals planned for dinner. Would it be healthier to eat nothing all day and 500 cals for dinner, or doesn't it matter at all?