How did you calculate to expect to mine .8 BTC during such a long period of mining (60 months)?

You buy some mining gear and with that you basically have a somewhat constant but limited hashrate (unless you add gear). Did you take difficulty adjustments into account? (I didn't do a quick napkin calculation to estimate if you've taken this into account; my gutt feeling tells me, you didn't).

Have a closer look how global hashrate evolves over past months and calculate a few scenarios for your 60 months time frame how global hashrate might develop (you can extrapolate the past into the future; block subsidy adjustments could pose some challenges, depending how Bitcoin rate develops). When your own hashrate is constant and global hashrate and difficulty rise, guess what that means! Hint: your mining gear earns you less and less coins because your hashrate proportion sinks...