The lumen meter is a cool gadget I would like to buy someday. How much was yours and what model?
I usually use my lights for 4 grows. I think any less would be a waste, and I have not seen any difference in yield and I grow in the same room with the same plants (clones). Some change them earlier and there is nothing wrong with that, it's just I'm a tight wad and really have to watch my coin these days.
WP