Измените формулу стиля xlR1C1 и используйте .Range("L" & i).FormulaR1C1 = ...
.Range("L" & i).Formular1c1 = "=(NETWORKDAYS(rc10, rc11)-1)*(r1c16-r1c15)+IF(NETWORKDAYS(rc11, rc11), MEDIAN(MOD(rc11, 1), r1c16, r1c15), r1c16)-MEDIAN(NETWORKDAYS(rc10, rc10)*MOD(rc10,1), r1c16, r1c15)"
Если вы не слишком знакомы с синтаксисом xlR1C1, пусть VBA сделает всю работу.
Sub Sample()
Dim i As Long, fA1 As String, fR1C1 As String
fA1 = "=(NETWORKDAYS(J2, K2)-1)*($P$1-$O$1)+IF(NETWORKDAYS(K2,K2),MEDIAN(MOD(K2,1), $P$1, $O$1),$P$1)-MEDIAN(NETWORKDAYS(J2,J2)*MOD(J2,1), $P$1, $O$1)"
fR1C1 = Application.ConvertFormula(Formula:=fA1, _
FromReferenceStyle:=xlA1, _
ToReferenceStyle:=xlR1C1, _
RelativeTo:=Range("L2"))
Debug.Print fR1C1
'=(NETWORKDAYS(RC[-2], RC[-1])-1)*(R1C16-R1C15)+IF(NETWORKDAYS(RC[-1],RC[-1]),MEDIAN(MOD(RC[-1],1), R1C16, R1C15),R1C16)-MEDIAN(NETWORKDAYS(RC[-2],RC[-2])*MOD(RC[-2],1), R1C16, R1C15)
With Worksheets("Storage NSNR")
With .Range(.Cells(2, "K"), .Cells(Rows.Count, "K").End(xlUp))
.SpecialCells(xlCellTypeConstants, xlNumbers).Offset(0, 1).FormulaR1C1 = fR1C1
End With
End With
End Sub