Hi , my question is should I run my humidifier all the time , or I must close it when I off the led light ? my humidity in the tent is 50%
The goal would be to keep the humidity where you want it for the stage of growth you are in. 65% for veg and 50% for flower is a good place to start.
I go by this: https://www.perfectgrower.com/knowledge/knowledge-base/vpd-chart-vapor-pressure-deficit/
Technically speaking, keep the proper VPD all the time would be ideal. However from a practicallity standpoint, it's not worth the hassle. I don't track my VPD in my grows, if I'm in the 60's in veg and mid to low 50's in flower, I never ever have any issues. For a new grower, VPD is not something they really need to dive into right off the bat. Too much information can be a hinderance sometimes.