Following is a link to a bitcoin calculator in Excel that I have been improving and maintaining for some time now. It is based on an original model created by the folks at Bitmain. The historical data is current as of the last difficulty change.
https://drive.google.com/file/d/0B1DkFD-cfVr4d2NkV2hyWTE2NVk/edit?usp=sharing
PM me with any question or enhancement requests.
Thank you for this. One thing that took me a little while to finally see is that this assumes that you end up with 3 Antminers by the end of the year.