The answer is “personal experience,” or more accurately, the experiences of our customers coupled with internal tests we have performed in our laboratory. We interview most of our customers to understand their pre- and post-implementation experience. The ROI calculator that we have developed is actually pretty specific to our platform and is very conservative to allow for any significant margin of error (we would rather underestimate your savings and be pleasantly surprised, rather than vice versa)
However, you should be able to use the tool when evaluating a variety of solutions as a benchmark of the savings you ought to be able to achieve.
If you are interested in discussing the calculator in more detail, we would be happy to set up a 30-minute phone call with you.