I think you're going about this the wrong way (aka looking good now, but causing you major grief down the road - database normalization). If you think of a manhole in terms of street intersection, you should have a field for Inv_Dir (just like in a street table, Str_Dir). An alternative would be to use something along the lines of a Dir_Ref field (Directional Reference). This will allow you to have a one-to-one relationship with your InvFrom and InvTo fields. Additionally, this will also allow you to use directions like "SW", "NE", "SE", "NW", etc.
Hope this helps.