There is even a special flashlight app for that: search for ‘ceilingbounce’. It makes use of the same phone sensors as the various luxmeter apps so in accuracy it is not anything special but it has all sorts of nice features that makes flashlight measurements more convenient.
edit: de ceilingbounce app is not in the app store, a link is here: Introducing Ceilingbounce - flashlight testing and runtime graphs for Android