Best advice is break the fast gently, i.e. juice, then fruit, fresh veg, and as Chrisb1 here said to me "DON'T OVEREAT!" He also mentioned food rotation, which I vaguely understand but have not really tried out yet.
After a fast, I continue to weigh myself every morning to check if I have put weight on, if I have put a lot on I know I have overdone it. I can then eat a bit less the next day. That's how I do it, when I am being really disciplined.
I think thought that you will eventually get to know your own body, and know how much you need. Of course, you benefit from the fasting experience as well, and after a fast or two you will find that you are really attracted to healthy nutritious food rather than any previous bad habits you may have got in to. So it all helps.