you need to extact the column b from first record. try below solution.
Note: This is not a tested solution as I at present I don't have CR.
create below formula Count
and place in detail section:
Local NumberVar a;
a:=a+1;
This will number the rows starting from 1 to the last row.
Now create one more formula StoreB
:
Share NumberVar b;
if {@Count}=1
then b:=columnb
This will store the value of columnb in first record in variable b
Now use where you want:
EvaluateAfter(@StoreB)
Shared NumberVar b;
(ColumnAFromCurrentRecord - b) / b
Let me know how it goes.