I just do calculation up to halving, then a brand new calculation that assumes a ~9mos (or whatever it is exactly) lead time for 12.5btc/blocl. Then add the two together.

From what I see though, breakeven is around the point of halving, after which there may be another 10-20% return over an extra 6-8 months.

Biggest concern is that bitcoin price could move faster than difficulty, as I think it's realistic to assume $700/btc or better by 2017.