Could be reasonable. In a diffuse transmission situation I think it should be Luminance (cd/m2) = Back Face Incident Flux (lm/m2) * Diffuse Transmission (\) / Pi\.\.\. if your 18W CFL have degraded to 1000 lumens each then 4 \* 1000 / 1 m2 \* 50 / Pi = 636 cd/m2. Maybe your light box is smaller than 1 m2 but the diffusing surface is probably less than 50% transmission.
Ideally you should try to find a luminance meter you can use to verify your camera calibration in photosphere.
Thanks Christopher!
That really helps; its confidence inspiring if you can post-rationalize data "manually".
I will do that, the idea of my camera being able to capture cd/m2 is just awe inspiring 
Thanks again,
-jelle