In the beginning I was putting them all in and ended up with a good number of escapees from the tank.... either through the mesh or even the edges of the glass door. It was frustrating. Putting glass on top (for humidity reasons) only helped some and I still found them often. This was when I had a pretty bare tank in the beginning and they would scatter in the moss pretty quickly.

Then I redid my tank (to make it more proper for my species no thanks to the pet store) and I ended up with not only leaf litter which allows for a flatter feeding surface, but a large piece of flat wood which is my chosen feeding spot. It works great, and though the flies do scatter some they don't get far before he gets them. In fact not many make it off that log. The ones that do he picks off throughout the day, and I haven't had escapee problems since. I also, once I get them into the cup I will feed from doing the first cup method and then tapping into second cup to leave behind the extra supplement dust, put a lid onto that one and only drop in about a quarter to a third of the flies at once and let him get most of those before I add the next round. It takes longer, but I like to watch him eat anyway, and from start to finish I am always done within 10 minutes.