Hoppy, you've been such a good help with lighting before, let me ask you something as you'd be better interpreting the graphs and giving advice than anybody.
I have a 29g tank, and am looking for low light, or more specifically, the highest light possible to get away with only using excel for CO2. For the room it's in, if I used one "good" metal vertical housing mounted about 3' above substrate, what bulb wattage would I need to hit my target? What if I used two housings? Finally, what if I just set a housing or two directly on the glass?
I know it's a lot of questions, but I figure you might know an answer in a minute, as opposed to me looking at the graphs for hours and ending up with the wrong answer anyway!
Edit: I figure I'd want to use 2 fixtures (to get better spread), in which case just tell me which scenario you'd suggest and what to get, as I'll do whatever you'd suggest.
Edit 2: The more I look at the one graph, the more I'm pretty sure that if I'm at 15" (on top of glass), I'll surely have too much light with two fixtures. And if I am 36" above (can mount to something at that height easily) I'll surely have too little light no matter what. So, I will find a way to mount at any height if you can tell me at what height and bulb wattage you think a fixture system would work best.