Hi Eric,
You can copy a custom form by selecting the check-box then the Copy button appears. I never noticed that until now :)
Another more complex concept which I didn't really appreciate until recently is that you can have the same calculated field on multiple custom forms, but with different calculations on each form. i.e. the data that results from the calculations ends up in the same place, but the calculation used depends on which custom form was attached. This allows for some interesting possibilities.
Regards, David.