Simple Math calcualtion using VBA programming

Hi Friends,

Iam quiet new to VBA programming,requesting for help

Iam performing simple math calculation using VBA prog

CTN QNTY CTN OUT CTN AVAILABLE
30 5 25

CTNAVAILABLE= CTNOUT - CTNQNTY

I want to implement a constraint
if ctnout is greater than equals ctqnty then
make ctnqnty=0 and ctn out=0.

currently am implementing the following code

Public CellCopy As Variant

Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Range("L:L")) Is Nothing Then Exit Sub
Application.EnableEvents = False
On Error GoTo ErrorRoutine
Target = Target + CellCopy
ErrorRoutine:
Application.EnableEvents = True

End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Intersect(Target, Range("L:L")) Is Nothing Then Exit Sub

CellCopy = Target

End Sub

Kindly update me with your answers as soon as possible.

thanks