What kind of light level (lux) during the dark cycle will cause my plants to herm in the flowering stage?

you do not want any light leaks what so ever, you want it dark, so go in bathroom when its dark and find light leaks, put towel in front of door and if you use that bathroom when the light are off and you turn them on is a no no. if your pick up some green bulbs that produce green light you can turn those on anytime during the dark cycle as the green does not affect the dark cycle, but any light will trigger the plant to go hermie on you and come harvest your plant(s) will get seed that will all be hermies, green light will be your friend in that room