What sort of depreciation models are you guys using for your miners?  When I first started I came up with an exponential formula to depreciate each miner monthly by 10% each month from it's previous month NBV rounded up to the nearest $25.  This would make each miner fully depreciated around 2 years which I figure is close to the working life of a miner.  I was also trying to find a balance between difficulty and BTC price, but since I came up with that formula the BTC price has exploded so I'm thinking of possibly changing me deprecation model and interested in what other are doing.