This is a very typical problem when building database apps - and it’s made more confusing because it is something which is generally easy to achieve in a spreadsheet!
Knack does not have the ability within an Object to sum values from other records in the same object unless the records are Connected (i.e. a connection between the object and itself). I have set up an object which illustrates this - hopefully the field names make this fairly self explanatory…
So February’s record is connected to January’s and March’s to February
Using this method, you can build a set of connected record within the object which will contain your running sum. At the beginning of the year you simply set up the first record without a connection to an earlier one.
Then there is the question of how to use this in practice from a user point of view. The simplest is simply to have the user manually select the correct Previous Record (a drop down filtered by the staff member would be best) - but there is another possible way which is to have a Record Rule which creates the NEXT record when you save the current months one - I have tried this and I have had an issue with the record rule which I have reported to support. I’ll update when I hear back.
There is a third way to achieve this which would be to use Integromat to calculate it. When the latest record is added / edited get Integromat to find all previous ones in the year, add up their figures and post the total as a running sum into the record just added. There are potential complications here if an earlier record is updated because it would the be necessary to update any record after that date.