- 1' of coverage at the water surface with Narrows (60º) implies a 10" mounting height, I think.
- The Wides (140º) would need to be mounted 2" from the surface to give equivalent coverage.

- It seems like 120" x 36" = 30 square feet.
- 16 Kessils set up for 12" diameter of coverage is (ignoring the circle/square thing) only 16 square feet.
I'm not saying you need more than 16, but does that seem to add up?
If you don't already have a light meter, get one. I'd say the thing about the lux meter costs $15 and how it's nice too, but with 16 Kessils I imagine you can swing the Apogee 500-series that is supposed to do a better job reading LED's– even better than the usual PAR meters you see around.
But whatever you pick, definitely get some kind of light meter to use during setup. You will save yourself a wallop of dissatisfaction and mystery and cut setup time.