Timbo,
I'd start by logging a custom event every time one of your players finishes a level, e.g.:
Flox.logEvent("LevelFinished", {"Level":"3"});
That way you'll need but one look at the associated pie chart in the web interface and you'll see where most players drop out. You can also tackle this the other way round and log an event, when a player goes game over, e.g.:
Flox.logEvent("LevelFailed", {"Level":"4"});
The corresponding pie chart will show the level with the most drop outs as the biggest pie.
Be sure to submit strings instead of numbers: Since you'll probably not have a trillion different levels, you'll want flox to show a pie chart where each level gets its own pie.