For ROI what should be the time span to be taken into account ?
Fix a certain % bitcoin difficulty increase, I will recommend 15% to 20% for calculations.
Put a standard electricity charge of around $0.15 per KWH. No need to take timespan into account.
Now find out the ROI % in BTC terms. This is a rough idea and you need to refine it as you progress into the project.
If %ROI can be spread over like this, one can simply look at the charts and select which miner will be more good to buy.
A variable difficulty will drop down box in your spreadsheet will be an added advantage in the selection process.
I was not including ROI only because the electricity cost is different for each miner and spreadsheet environment is tricky in a way to be used by multiple people if they change something. But yeah, the sheet will be complete if we have ROI figure. Let me sleep over it.
