Thanks! I was starting to think it would make more sense to simply sanitize the input of the user when those cells were changed, but your approach seems like it would work really well!

Now if to generate that formula in VBA, when column E is changed. Is there any simpler way to do that than what I've outlined below:
Dim f as String

If Target.CountLarge = 1 Then
If Intersect(Sheet1.Columns(5), Target) Then
f = "=ISBLANK(" & Target.Offset(0, 4).AddressLocal & ")," & """ & """ & ",(INT" & Target.Offset(0, 4).AddressLocal & ") + MOD(" & Target.Offset(0, 5).AddressLocal etc.

Target.Offset(0, 2).Formula = f
End If
End If
Cause getting the formula correct is going to be something of a pain, and very hard to read for whoever comes after me. Dunno... I have something of a start at least, even if I won't know if it works till I get to work tomorrow

Again, thanks for all your help so far.