It's easy to do by adding an extra column. In that column you would keep a running total by filling down the a formula like this
Imagining your data has a header row in row 1 and is in A1 to C6 put this in D2 and fill down
=SUM($C$2:C2)
Then in E2 put this
=COUNTIF(D2:D6,"<500")
Changing the number 500 will give you a different limit.
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