I have to agree with a few others here. If you are after larger lettuces, then spacing them 10 inches apart is ideal. However for us we sow close together and get good results although smaller lettuces.
Sometimes the smaller leaves are tastier
We have Great Lakes and May Queen. The Great Lakes are very tasty and don't need much space at all. The great lakes need a little more space, probably 4-5 inches to allow them to spread a little.
Robert