light output at 50 percent

Maybe, insufficient information. Hanging distance -- further away gives better light penetration and more even distribution of light wall-to-wall... there are limiting returns on that and a little common sense required to interpret and apply this concept properly. But, the context should answer 'how much distance' ... As far as initial power target.. work backward from DLI. In ambient co2 conditions 35-40 dli is a good atarting point. From there, 600ppfd over 18 hours and 900 ppfd over 12 hours. PPFD X M^2 = "umol/s PAR needs"... (working backward... if going from the light to size it for a tent.. umol/s PAR / m^2 = ppfd, and target ppfd depends on hours of operation and expected DLI) "umol/s PAR needs" / total umol/s PAR of light = percentage of power to start. These are ballpark numbers... A good starting point from which you will need some minimal adjustments based on observed growth. Closer is a great idea for shallow canopies -- SOG or early vege growth, for example. Don't try to minimize hanging distance and watts if you are growing 18-24" deep canopies. What is best depends on context... also, consider exactly how much you are saving.. if it's a matter of 2 dollars over 2 weeks, is that worth it? e.g. i could get my vege lights closer and maybe save a kw-h per day, which is only 20 cents. Or, i could endure that cost and benefit from the fact my light is only 10-15% less intense in the corners compared to central reading. For 3 dollars over 2 weeks of early vege, i get significantly more even coverage across canopy. I didn't use real numbers here, but i did just have a similar choice... I chose the slightly more wasteful route because it was 2-3 dollars difference over 10-14days when i worked it out. The point is, one-size-fits-all doesn't work here... you need to consider each context and allow that to dictate your best options. Don't get too hyper-focused on one aspect and lose sight of the bigger picture. On top of all that, each person values these various aspects slightly different from the next, too.