Hi Eileen,
I think you are on the right track, to most used measures that I have heard people used are FTE savings and/or hours returned to the business (the latter being what you have landed on).
Some customers do the same as you, tracking what the human handling time was for a process that humans no longer worked. This becomes more difficult to track for the biggest RPA capabilities so a Robot to Human equivalent is sometimes landed on by using historic data and then extrapolated for future automations. So, if you your existing robots on average do the work of 3 people - going forward you could use robot utilization data to extrapolate hours returned.
The one risk you take at purely looking at hours worked equivalent is you will be missing a lot of additional savings that your current measure will exclude. For example, if an automated fraud process does more work than humans previously had capacity to work, and this has lead to a decrease in fraud - are you including those £$ savings from less fraud in your benefits calculations. Ian Hardy wrote a good summary of some of the additional benefits measures you should probably be capturing here:
Uncovering the Hidden Benefits of Intelligent Automation | SS&C Blue Prism
