Excel 2007 :: If Then Else And Validation

I am failing on the logic and syntax to achieve the following. Basically it has to take the sum of 1, or 2 fields and validate it against the value of a 3rd field. I can get it to work for 1 field if DefPercent = 100 but not when looking at the total of 2 fields if DefPercent < 100.

Fields:

- DefPercent (if = 100 LiabilityAmt2.Enable = False) (this works fine already)
- TotalLiability
- LiabilityAmt1
- LiabilityAmt2

Output Required: logic only

If DefPercent = 100 And TotalLiability LiabilityAmt1 Then
MsgBox "error....."
Exit Sub
Else continue

If DefPercent < 100 And TotalLiability LiabilityAmt1 + LiabilityAmt2 Then
MsgBox "error...."
Exit Sub
Else continue