For a bike like the Cypress I just like to be able to bracket my size by trying the sizes on either side of the one that I end up with. It's like when you get your eyes checked. If going smaller feels worse and going larger feels worse, then you've found the right size. From there you can dial in the details like stem length, handlebar rise, etc.
I saw your other thread about having bought the 18" frame size. Does your shop have a 20" frame size in stock for you to try? If you are in doubt, the ideal would be to try both sizes of the bike and decide for yourself which one felt better.