Glad to see so many have found this useful. I haven't even looked at it in months, I didn't' realize that it was so popular here.

Remember that this is an open sheet that any can fiddle with, so check all your Mining Factors in the grey box and make sure they fit your particular scenario. This means how many Mega Hashes your equipment is running, how many Watts you're using, the cost of electricity (if any) for you, how much you think the BTC will be worth when (and if) you sell them, ect...

Also if you want the most accurate difficulty forecast, enter the most recent ACTUAL difficulty and the date it started.