35 to 50w per sq ft for flower is the way to go.
As Nope said, I'd keep those lights for cloning/seedling/veg and get more intense light for flower.
Clarifying question, when you suggest the calculation begins with 35-50w per sq feet, do you mean length/width only? Or the total volume of grow space which includes height?
My grow space = 5ft x 2.67ft x 6.67ft. Using all three dimensions = 89sq feet, vs just WxL = 13.35 sq feet.
Which number do you use to complete the math?
For the sake of simplicity I used 40w x 13.35sq = 534, or if its 40w x 89sq = 3560.
When looking at grow lamps some boast 4000w, but they look cheaply made and I doubt I need (or they produce) that many.
Other folks suggest on this forum state I need close to 450-500w in total for my space, so I see potential value in either calculation. Thoughts?