on design view of your form, add code to the ActualWeight textbox BeforeUpdate event:
Code:
Private Sub txtActualWeight_BeforeUpdate(Cancel As Integer)
' put the estimated weight here
' or put it on the table
const EstimatedWeight As Double = 1000
dim TenPct As Double
TenPct = EstimatedWeight * 0.1
Cancel = [txtActualWeight] > (EstimatedWeight + TenPct)
If Cancel
Msgbox "Weight is more than 10% from " & EstimatedWeight
Exit Sub
End If
Cancel = [txtActualWeight] < (EstimatedWeight - TenPct)
If Cancel
Msgbox "Weight is less than 10% from " & EstimatedWeight
End If
End Sub