Ok I got something that works, used a simple IF statement.
Private Sub Text1_AfterUpdate()
If IsNull(Me.Text8) = True Then
Me.Text2 = Format([Text1], "yy")
Me.Text3 = DMax("[Rec# part 2]", "Table1", "[Rec# part 1]= '" & Forms!Form1!Text2 & "'") + 1
If IsNull(Me.Text3) = True Then
Me.Text3.Value...