Here's maybe the original PVC lightbox posting: https://budgetlightforum.com/t/-/6918. More info there.
I would no way, no how claim my measurements are accurate to true lumens, but I also know it's damn close, in the neighborhood, shows consistency over and over again when done and setup correctly. I see lots of consistency from unit to unit (many TN31's and many Shockers for example). Also I've done many battery tests in various well regulated lights and poorly regulated lights and can easily track differences in lumens output as a result. Best part is making a tweek and seeing the measured result, and also, getting someone else with a totally different home made lightbox, calibrated completely different and getting the same results you get - amazing coincidence that seems to happen quite often...