A few additions: lumens per sq ft means just that. It is not how many lumens are emitted, it is how many reach the target area. So for starters you have to divide the total lumens by the number of sq ft. Then, you have to do an honest estimate of how much light is realling falling on the grow area. Is any at all hitting floors and walls? Of course it is, and 3000 per sq ft minimum doesn't include what is lost. So you really ought to shoot for 5000 lumens generated for every sq ft of grow area, and regard that as a minimum. You can get by with less in early veg, and you really should have more in flower. Both in the correct spectrum ratio as well.