Given that there is a difficulty adjustment every 10.7 days now, any calculator that assumes steady earnings/difficulty per month is very wrong.
Absolutely! Because of that fact the chart is based on the difficulty adjustment periods.
But the table above isn't - it's the classic calculation with an assumed fixed difficulty. For one Day that is ok and for a Week may be - dependent of the time left until the next difficulty change. For a month the prediction it's definitively wrong.
Hmmm ... may be better to change the table to display the next three difficulty adjustment periods? I will think about that ...
Thanks for comment
Volker
... just updated: http://btcmc.de
