Take last data of a table

Hi guys.

I have 500 sheets and i'm using this formula in A11 =index(A1:A10,counta(A1:A10)) to take the last data entry in a table in every sheets. But the problems is that the answer didnt appear because i'm using this formula, =if(B1="","",C1+C2) in A1:A10. So, to edit it back will take a long time. So, what i wonder is that, is there any way to make vba to take the last data and then paste it below the table probably in cell A12 in every sheet? I already attach a simple dummy file. Hope someone can help me..

Thank you