I’m starting to see multiple white hairs popping out on the brackets. I’m assuming that I am in the pre flower stage. The manual for my light says 18-24 inches for veg and 12-18 for flower. Should I move it to 12 inches since I am in pre flower?
I would start at 18 and ease our way down.
What height are you now? 
Light intensity is measured in PPFD (photosynthetic photon flux density.) You can get a phone app called Photone to measure the PPFD at the plant’s canopy. I run 1,000 PPFD during flower in my grow.
DLI (daily light integral) is another way to measure light intensity. DLI incorporates both PPFD and the number of hours of light per day that the plant is receiving.
24 inches
I’d start here and see how they respond. I’m loathe to put my lights closer than that.
What type of light you got?
Spider farmer sf1000d
I’m with @Graysin 18 inches is the closest I like to run my lights to plants. I’m usually more around 21 inches.
Good luck
Photone app works wonders for me with DLI. No measuring distances.
Spider Farmer’s recommendations for hanging height are usually the range to stay in. Like @MidwestGuy said DLI is the part the requires the most dialing in. Stay in the recommended range for hanging height and dial in DLI with the dimmer. Until I started using Photone (luckily I started very early into growing) I was basically just guessing.
My light does not have a dimmer.
If you have someone else to help you raise and lower your light, then you can just hold your phone above your canopy while they raise and lower until you get the right DLI that way not a lot of ipdown
Looking very happy and healthy! 
What should the DLI be?
