That is a tricky set-up, jon! To have a fixed calibration for all kinds of flashlight sizes and types, your set-up depends on a luxmeter that correctly reads light coming from any direction, hitting the detector at any postion (and as I understand, that is not an easy requirement to match for a luxmeter).
But the proof of the pudding is again in the eating, if in a series of tests, the calibration of this set-up proves correct enough in various types of flashlights, all is good. 
Edit: TA had a parallel answer to mine .