Not quite...
You don't need the IFs - the CASE does the IF work for you.
Switch $WindowSensInput
Case 1 ;Checks if $WindowSensInput = 1
$ActualWindowSens = 0.031
Case 2 ;Checks if $WindowSensInput = 2
$ActualWindowSens = 0.222
Case 3 ;Checks if $WindowSensInput = 3
$ActualWindowSens = 0.333
; Case n ;Checks if $WindowSensInput = n
; $ActualWindowSens = whatever
Case Else ;Checks if $WindowSensInput = any other value not stated
Msgbox(16,"Error","Your value is outside the range allowed")
EndSwitch
