Diff. Increase per day: 1.3753 %
The next difficulty increases is now 30%+, if you compute based on your price, 30% diff increase and the end of August mining start, ROI by mining will never happen.
Looks like that is already built into the calculation of ROI in the form of a predicted daily difficulty increase.
If you mine last week of August, the Initial Coins per Day is around 39 not 65.90257387, so its a different result.