Try:

=(8.5-SUM(B$2:B2))/MAX(10-A2, 1)

or

=IF(A2=10, IF(AVERAGE($B$2:B2)<0.85, TEXT(0.85-AVERAGE($B$2:B2), "0.00%") & " short", "success!"), (8.5-SUM(B$2:B2))/MAX(10-A2, 1))

Then copy the formula down the column.