You are correct in that you would need to do this in another way, as its not possible to have the ITAM UI do this directly. Using the Web Services is one way. Now, depending on how fast you need the update, and how data gets entered into the system in the first place, and how this field gets updated/recorded, you have lots of options.
If you want to attempt PAM to do this, then you would need to create an event that fires on that field in ITAM, and have that event call your custom process, and have that process do the calculations and then update the asset with the new data.
If you only set this field during asset loading, perhaps its better for the program that does the load (or creates the file to load) to do the calculations and have your import process take care of it.
You could in theory also write a background service type of application (via Web Services), which nightly scans the assets for changes to that field, and for any that have changed, does the calculations and updates those assets...
Lots of options, but which is the right one for you, I can not say.
Rick