another idea might be to just look at some light manufacturers specs on their growlights...... a 400w HPS light puts out something like 60,000+ lumens and is recommended to illuminate a space approximately 2x3'...... 60,000 lumens divided by 6sq/ft = 10,000 per sq/ft..... and so on for larger lights.
These numbers are also related to Flowering..... much less light is needed for seedlings, mothers, and vegging...... plus alot more plants can share the light at the lower stages of growth