ROI is easy to calculate, but changing very fast.

For example I calculate my profit before 2 months for one 1070 card as 45 USD - 20 USD for electricity, but was after month 60 - 20 USD (Bitcoin go up for 30 percent and took other currencies with him).

So you may calculate some "starting" point, and definitely is good to think about revenue, but reality will be different after one month and will variate heavily by tens of percent after several months... Its just my 2c.