How far should i keep my 600watt MH light from my 2 week old plants?

I have 4 plants in a 4×4 tent, in 5-gal smart pots, in Happy Frog soil... Under a 600watt VIVOSON MH light with a wide hood & with a glass shield, and I have a carbon filtration system & fans...
This is like my 5th grow, so I've got pretty much everything down, but my last grow produced AIRY buds on all my plants...
So I know it's a light issue...
How far should my MH light be away from my plants in 1st week of veg & then maybe someone can give me some better light practices... ty

I'm also using a 600w vivosun MH, and my 4 plants are roughly a week old in 5 gal grow bags as well. How I measure the distance of lighting is by putting a Gove wireless thermometer at the level of the canopy and I can check temps and humidity from my phone. I lower the light until I achieve 78-81 degree temps. Too hot I raise. In my opinion, airy buds are from lack of light during flowing stage. Right now my light is 46" from the canopy