I finally figured out how to find the center and it makes changing the centerboard a snap. I made a paper copy and triangulated it smaller and smaller until I reached the smallest size.  Stuck a nail in the center of the smallest triangle mark and bingo.  I drilled a hole through the board at about 3/8 up from that point to allow it to just hang in the right direction yet be easily moved. I put a 5/16 bolt thru and hung it on 2 x 4 each side. Now I can side the board on my bucket dolly to the exact spot and gentle slid it up and down to get the holes to match up. Works great and with just one person makes the board easy to put on and off from now on. Man handling that board off the boat the first time was a night mare.