Hey evilmobo ignore my previous figures because they are incorrect. In the previous calculation I said 7.5 when it should of been 2.5 Pop22's table made me see the error of my ways lol ...see adjusted calculation below
54.6watts x 2.5ish (umol/w) = 136.5 or rounded up 137umols/s per COB
12 (COBs) x 137umols/s = 1644umols/s per 1.2m2 = very good - maybe a tad high for a 1.2m2 area.
I still think you can get by on 10 COBs@1370umols/s and probably 9 for 1233umol/s
For a veg set up I reckon 6 COBs per 1.2m2 area will take you to the uppermost limit of where you want to be, so it still might be worth you experimenting with 4 COBs to see how you get on.