Found a solution, for any that may come after me :)
Dim HasDecimal As Integer
HasDecimal = InStr(1, Me.Hours, ".15" Or ".30" Or ".3" Or ".45", vbTextCompare)
If HasDecimal <> 0 Then
MsgBox "Hours must be entered in 15 minute increments using .25, .5 or .75 values. Please try again."...