Todd I used 8 in centers as do most commercial growers. I will tell you though due to the fact I am not harvesting the whole plant and just cutting off the outside leaves to eat I had to thin the plants out big time. I think on my next raft I will use 12 in centers. Keep in mind my system is a hobby system so it is a cut as you want to eat thing. The lettuce is on steroids to say the least. Plant selection will determine your spacing as well. Cabbage needs more room than spinach if you get my drift. Decide what you are going to grow and drill accordingly. Trust me after your first harvest your raft will change to meet your diet and quanity of foods you like to eat. I have way to much lettuce but wish I planted more cabbage for example.
Todd I have to agree with David, "Trust me after your first harvest your raft will change to meet your diet and quanity of foods you like to eat." I also made some templates which helped a lot. I'm doing some mixed rafts, e.g. Taro along with Basil and chives, I have some pictures posted. I'm also using a variety of netpot sizes so that too dictates the spacing I'm using. I have troughs 4' x 8' with the raft cut into 4 4'x2' pieces each of those pieces hold 18 2" netpots quite comfortably for lettuces however if its tomato or ochro the spacing would have to be a little wider to allow for the width of the bushes when they grow so I space the pots around 18" apart. Hope this helps.