Adding two numbers together using an automation instead of just being text

Ok, so I got it working. Since Part # is a text field, and I only want it to assign a Part # if the user puts "todo" in the Part # box. I made another field that, like you said, is a Formula-Numeric field called "This_Part#". It then does the calculation. Then the automation will check every time an "Inventory" is added or modified. If it was then it checks if Part # = todo. If that checks out it then sets Part # to "This_Part#" This is how I did what you said. Is this a good way of doing it or is their a better way?