Joshk, is it okay to post this here? I would think it’s okay, but if you object, I’ll delete it.
Texas_Ace is going to build a limited run of calibrated Lumen Tubes/Spheres (with no math involved!) for those that want to measure their lights output. It seems capable of measuring at least up to 25k lumen. More details are here.
He built his original Lumen Tube based on measurements he got from his Joshk sphere. Then refined his tube with lots of trial and error. So if you missed the Joshk sphere sale, you might want to check this one out.