Hi Ervin , have you tried ceiling bounce tests?Mine is in my bathroom , no windows , with the lens 20 cms above my washing machine lid(SI standard 1246577013124732kjdsbffhfuiezq) 
.If your box is no more accurate than that….rip it up and throw it away |(
Seriously ,; I am getting 25 lux per lumen on my very amateur(and cheap) sphere.That is 25000+ lux from my 2 EE X6’s with LG cells (HE 2 and 4.35v) at 1000+or- lumens.Djozz is getting similar results post 38 .The Pipe dream guys see post 47 , are also getting low candela readings.That means a low light sample so inaccuracies (say between warm and cool tints) may well be highly exaggerated.