Inconsistent mining profitability calculations
The problem is that you're not using the correct conversions:


  The reason I am dividing difficulty by 100 above is so I can keep using my hashing power in TH/s


There are 1,000 GH to a TH.


  difficulty / hashrate[GH/s] = time[s]


difficulty and hashrate stand in the following ratio:

difficulty = hashrate[H/s] / 7158388.055


Note that the formula uses hashes not gigahashes.