Hello Gary. A wishing well, that is a nice project to do for your wife. If the plan is calling for 10 pieces of 2x4 for each layer the 18 degree cuts is correct. The formula is 360 degrees for a full circle. If there are 10 pices meaning 10 sides that there is a total of 20 mating pieces. Dividing 20 into the 360 give you a cut of 18 degrees on each mating end.
Sounds like a nice project to do. Please post when it is done. Happy cutting ther Gary and don't forget to toss in a few good luck coins for a rainy day.