Just Google it. https://www.trollingmotors.net/blogs/selection/86935495-shaft-length-guide

   Yeah, I saw that.  But it measures to the water, not the bottom of the boat.  Has adjustments for weight, chop, etc.  Doesn't calculate length so that the top of the prop is 3-4" below the keel/skeg. Even Minnkota Customer Service said that "hopefully" I'd avoid cavitation with that method. I just wondered if anybody had bought one and had first-hand information.  Thanks for the reply!

   Did you see the position of my outboard prop in the picture on this line........?      https://www.clcboats.com/forum/clcforum/thread/51607.html Cavitation?  Not a problem. 

I think you are thinking too hard on this problem.  You can work out the measurements you want and see if you can get the motor you think you need.  The thing to remember is that this is a sail boat hull. Not a row boat. Not a planning hull skiff. It is a sailboat, John's passion, if I remember correctly.   Under power it tends to squat regardless of motor trim.  A longer shaft might help, but its the dog's motor.     To fast and it shoots up water through the dagger board well.  I put a plug in it.  Weight trim is important. 

The main thing is the shaft length is between the bottom of the control housing (handle) and the top of the motor housing.  (as seen in the link) 

PS.....I've rowed the JS II  five miles on a canoe club night paddle. Power is better.