I usually gain back about 4-5 pounds right away once I start eating again which seems to be normal for most people. So I usually think that if you want to lose 10 pounds, you need to lose 15 while fasting.
It's really important to have a plan for after your fast. I am starting to think that this is almost more important than how you do the fast itself. You have to have a game plan of how you will reintroduce foods back slowly into your system--it's so easy to kill your results if you just start eating everything in sight.