I don't see where the specified 0.098 cent electric for the "optimal S5 miner" part is figured?
I did a rough calculation last week, at 2% diff increase, and came up with ballpark 250ish days to pay back the initial .666 BTC and a small profit.
That spreadsheet doesn't care about electricity price. All it does it figure out how long it will take to pay back .666 btc with 1 TH/s of mining power based on how many blocks Antpool should be finding per day.
As of right now, it will take roughly 80 days to pay back .666 btc on a PACMiC contract with a 5% difficulty increase.
A S5 will become unprofitable after roughly 120 days.
That is a 40 day difference, for me that is under the 50 day difference I try to keep it over and as such, I usually turn off the auto rebuy at this point. But, I think the price will rebound and the difficulty adjustments will be low for another 2 or 3 rounds before we see some bigger growth, so I will leave my auto rebuy on for now.