Depends on variety, but 1500 to 2000 lumens per square foot is fine for vegging, but up to 3000 can be used by the plant. During flowering, you can do more, like up to 5000 lumens per sq. ft.

Also, 18 inches even with a sealed hood cold be heat stressing the plants. I know mine did, even though my thermometer said it was only 86 degrees at the canopy height, until I put a small fan in the middle blowing straight up at the light. Now I can get it about a foot from the plants. I don't, because the plants on the edge don't get light that way. Also, when you start getting close to the maximum heat and/or light that your plants can handle, the more perfect your PH and nutrients must be because they have a much greater chance to cause problems.