Modules & VBA :: Show Pop Up Message Depending On 2 Fields Value Differences

I have 2 fields on my form service interval combo box and vehicle mileage text box I m trying to create a pop up message on there values

if service interval is 12,000 and the mileage entered in the vehicle mileage is over 12,000 then show pop up message.

this works but I'm sure its wrong don't no why I need the -1 anyways here's what I have

Code:
If Me.Vehicle_Mileage.Value > Me.Cboserviceinterval.Column(1) - 1 Then
msgbox "test"
end if

Now my problem what I can't get it to work . I still want the pop up message, if say the service interval is 12,000 and mileage entered in the vehicle mileage is 1,000 miles below the service interval I still want the same pop message but if 2,000 or more below service interval then no pop up message .

I have tried a number of sequences with no avail....