I lose the most when I eat however many activity points that I've earned. I never eat my flex points, and if I do, then I try to get enough APs to make up for them. I do sometimes save some APs for another day if I don't feel like I need them on the particular day that I get them.
I tried a lot of different setups before i settled on this one- some people have more success when they eat all of their flex points, but I'm not one of those, sadly!
Best of luck