[marcosdumay]

What I got after messing with it a bit:

The "cover costs" assumes your people will use the tool 8 hours a day. By the productivity you gain, it calculates how much you save in salary.

The "free up an hour" also assumes your people will use the tools 8 hours a day.

The "cover costs with" calculates how much your people must use the tool for it to eventually pay for itself. If it's larger than 8 hours, then the tool will never pay up.