If you know scripting I'd suggest just using a script for this as it'd be quicker

If not, you're going to need a fairly involved automation setup.  You're first going to need to create a linked field to the same table, and have an automation that'll trigger for each record when the "Benefits" field has been fully keyed in.  That automation's action would be to find the second latest record in the table and link the triggering record to it

In the table you'd now need to have a lookup field to display the linked record's "Benefits" value, and a formula field to add the "Benefits" value to the lookup field

In the automation, after linking the records together, you'd have a "Find Record" action to grab the updated data from the second latest record in the table (to get the updated formula field value), and then put the value of that formula field in the field of your choice

Thanks Adam.

I don't know how to use scripts.

Do you have an example of the second option?

I have already seen this possibility in Airtable Community but I have never been able to set it up.

Thanks,

Benjamin.

I'm afraid not, sorry! 

It looks like you want a running total. One of the examples in the Scripting Documentation is for a running total.
Note that this script works best in Scripting Extension, and run from a button. As written, it will work as an automation script only if there are a very limited number of records in the view.

Overall, I do not recommend using a non-scripting automation for calculating a running total. Non-scripting methods tend to need a lot of automation runs.