Here is my line of code (do not hesitate to tell me if
you see a more efficient way)

ActiveCell.FormulaR1C1 = "=SUM(R[6]C[-11]:R[70]C[-11])"
Range("Q8").Select
Selection.AutoFill Destination:= _
Range("Q8:FY8"),Type:=xlFillDefault

I will try your technique (formula, not formulaR1C1)...

Thanks for the fast reply