This may have been hinted at already, but...An old timer offered me some advice a while back. It was to make one glue joint at a time. We were talking about the table I made for my kitchen and the trouble I had in keeping the top flat. I made a couple of cutting boards, following his advice, and found that he was spot on. For my next table top, I will join two boards. Then the third. Then the fourth. And so on. It is easier to control one joint at a time than four or five.