My go to configuration is now 400 watts in a 2x4 area using 2x 45" 5.88
That adds up to about 63 cm square per watt. With fans, it does not even get warm. If my fans fail, there is enough heatsink for it to run passive until I can get to fixing it.
I only run it at its full 400 watts the last 5 - 6 weeks of flower thou.
A lot of folks look to build cheaper but I figure the extra money spent is more than recouped in the first harvest anyway AND you have a long term rock solid light you will be glad you didnt take any shortcuts on later.
The light on the table now:
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Its about 400 watts using cxb3070 but it would be the same light with 4 3590s on each.
Also, grams/square ft is more important to me than grams/watt but I still like to keep efficiency above 50%
Check out this guys grow. He does it the same:
https://www.rollitup.org/t/six-cxa3070ab-2x2-5-gorilla-scrog-grow.862912/