Well cubic feet doesn't matter.
You are worried about square feet. The light shines a certain distance to the canopy .That distance affects your light availability , i.e. closer=more lumens(because it spreads out, and the light is less "concentrated").
So, anyway... your light shines 12" down to the canopy and puts out 18000 lumens/(1.1 x 2.1)= 7700 lux
Or, more likely it shines closer to 6" from the canopy, this allows less spread and your lumens go up to 72000 lumens/(1.1 x 2.1) = 36000lux.
With a scrog, we are only worried about the canopy, but with a regular grow,the cubic ft becomes more important, because you need high lumens further away. For instance, 2 ft away from your lght, you are down to 4500 lumens/(1.1 x 2.1) =1900 lux.
Google the inverse square law calculator, to help understand what I am saying.
You are worried about square feet. The light shines a certain distance to the canopy .That distance affects your light availability , i.e. closer=more lumens(because it spreads out, and the light is less "concentrated").
So, anyway... your light shines 12" down to the canopy and puts out 18000 lumens/(1.1 x 2.1)= 7700 lux
Or, more likely it shines closer to 6" from the canopy, this allows less spread and your lumens go up to 72000 lumens/(1.1 x 2.1) = 36000lux.
With a scrog, we are only worried about the canopy, but with a regular grow,the cubic ft becomes more important, because you need high lumens further away. For instance, 2 ft away from your lght, you are down to 4500 lumens/(1.1 x 2.1) =1900 lux.
Google the inverse square law calculator, to help understand what I am saying.