Is there a way using some code attached to my code given below that will
carry out this formula

=IF(P19="L",$S$6*$Z$1,IF(P19="W",($R$4*5)-$R$4,"0"))

in the cell directly under the cell the user input a value into

eg

The user inputs a number into cell P19, my input box will then be displayed
and I want the formula above calculated and the result placed into cell P20
as this is directly under cell P19!
my code thus far
Code:
Sub Input_box()
UserValue = InputBox("Unit Value?")
Cells(1, 26).Value = UserValue
End Sub

thanks in advance