Love the beta version - really looks great and you can see the potential this has when weighing things out, anyone serious (or thinking about being serious) about mining would be foolish not to pick it up.
Anyway, right now it's specific to the S5, which is fine, since that's what I have, but what I would love to see is another tab that would be Equipment, and on that break out several different devices. Imagine each column is a different device, and the rows would be:
Name
Cost
Hash Rate
Power Consumption
GHash/Watt (derived)
Amount (number of devices)
This way you could have columns for S1, Neptune, SP20e, etc - you would fill in the amount of each type of device you'd have, and then it would fill in Revenue sheet, which would be just like your Investment sheet, but instead of deriving devices from a Target monthly income, you could see stats related to actual operations of a diverse deployment.
One other thing you might consider is all of the secondary costs these devices have. For instance, on a basic model I built before, I also include PSU's, PDU's, Racks and Switches - those are pretty much the key things you'll be buying. I also included a number of devices per type; so is was 1 PSU to 5 S5's (I use the IBM 2880w power supplies), 16 S5's per switch (24 port switch, other ports for uplink and PDU), and 10 S5's per 30A PDU (240v). When you start adding these up, they can gobble up a fair bit of money, although most (if not all) will be fine for the life of the mine, as opposed to the miner that will be upgraded at some point.
One other thing you'll probably want to consider for the longer-term forecasting will be the reward halving that will happen next year, as it will certainly affect the economics of mining... You also have the average difficulty increase, but you might want to add something to account for bitcoin's value also rising over that period.
Which brings me to my final suggestion - the same way you modeled out Network Difficulty, it would be super awesome if you did that for the price of BTC. This way you could at least maybe make some long-term predictions - all of this is guesswork, but even a guess is better than no idea at all.
