Check Multiple Cells, Fire Macro Based On Result

I have three cells where a user will input data, in some cases (2T Weld Condition) they will only enter in B12 and C12, but in the case of a 3T weld they will also enter data in the D12 cell. I then use a formula to check for the thinnest material and that is entered into another cell with a formula, B14. I then need to check the value in B14 to verify if it is above zero, but below 0.65 (mm). If it is then I would like to have a message appear on the screen notifying the user that they are outside the acceptable range.

I cannot figure out how to use the information in cell B14 because it is a formula and my code only works with a direct value. The code I am using works if I point to one of the three input cells, B12, C12 or D12. How do I use the information in B14 to work with the code below.

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$B$12" Then
If Target.Value < 0.65 Then
Run "MyMacro"
End If
End If
End Sub

MyMacro loads a userform with buttons, etc.