Shane Devenshire wrote...
Try this, sort of...

=A2-MIN(MAX(30-SUM(A$1:A1),0),A2)

here I assume there is not number in a1 and that the data starts in A2. *
Fill down.

Or just make B2

=MIN(MAX(SUM(A$2:A2)-30,0),A2)

and fill down. This doesn't depend on A1 being empty.