36ish inches is a safe guess.The other thing to consider here if your talking a single 600 per 4x4 is "vertical coverage"
For those who have used vertical 600's, how many inches of vertical canopy can Mobo ecpext to have good light coverage on without moving the lamp up and down with a mover etc?
As we see @zep_lover has his lamps double stacked
But who's to say......i may be ok with those smaller nugs at the bottom on a 48 inch canopy......he might not want them.But after 6 years straight of a single 600 i'd go with 36ish inches of decent usable light.