exelent calculations, so this means you have found a very good way to earn out of mining, which miners do you use ?
GPUs :p
I assume you are not going to be mining bitcoin with this setup (likely will use a multi pool with scrypt based coins or some other algo).
I would not at all be surprised to see the difficulty increase as a faster pace this winter as people start to use their ASICs that are no longer energy efficient enough to mine more BTC then they cost in electricity again to help heat their home.