It's nice - but I find the BitcoinWisdom mining calculator much better still. Yours only calculates your earnings after the next difficulty increase. In reality, a miner needs to be able to model their returns after each difficulty increase. Without taking that into consideration, a miner will have an over optimistic expectation of return. Also - you should factor in the initial cost of hardware as well. With a couple more updates, I think this would be great!
I agree with the sentnence in bold, but I don't think that Difficulty Increase were not taking into account there. I mean I don't think that more complex calculation will change the picture to such an extent that we should worry about it.
Overall good job OP! Thank you for the simple and in the same time pretty accurate Calculator!