sharepoint online – Using a value from previous row in a new list entry

Like Siddharth suggested I think your requirement can be easily fulfilled using calculated columns. To do this click on add column and select more then on the new screen select calculated. Rest is pretty basic stuff You can see the attached screenshot for more reference. Once you have done selecting all the settings click on OK.
and Viola you problem is solved :]
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The new column would look something like this
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Hope this helped somebody out there :]