Just found a second hand KingLed 800w full spectrum COB with dimmer. I think I'm gonna invest into that. My box is 40cm x 40cm x 70

Just get a decent light bro there are lots of options out there ...50 watts aint goiin to set the world on fire!!! In my opinion full spectrum lights are better the ones closest to day light ...happigrowin!!

You didn't say how big your grow box is but my guess is that you could use more light. Maybe you can get your light closer to the plant while making sure to keep an eye on it for signs of too much light. Also, it makes a difference what you are growing. You can get away with lower light level if running the light 16+ hours a day but if on a 12/12 schedule having more horsepower on tap can really help yields. There are many websites that recommend lighting levels. Ignore the "equivalent" number for your light and go by the actual wattage it draws. Also don't sweat the hardware too much if this is your first grow. I tend to think experience and paying attention to what your plant is saying are some of the most important ingredients to successful farming.