Does each form submission contain a piece of unique data that you can use to tie to the previous form submission? With your example of vehicle miles perhaps you'd be tracking the VIN number and so you could use that?
If so, I would try:
1. Create a new table where each record represents a single VIN and link it to the form submission table
2. Create an automation that will trigger when the form is submitted and link that form submission to the appropriate VIN 3
3. In the form submission table, create an Autonumber field
4. Use rollups and lookups to identify the latest and second latest form submission per VIN
5. Use those lookups in formulas to get the difference between the latest and second latest miles submission
You can then use that formula field to power your automation


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