How do you create a running total using a number series rather than a date?

Thanks for the reply, aj1973! Unfortunately, changing that relationship to bidirectional didn't do the trick. The only semi-workable solution I've found is to change the code from using the Child table to the Parent table, which enables the visual to use the Month No. field from the parent:

Does work...but only with fields in child (i.e., Month Sequence):

Does work...but only when using a parent field (i.e., Month No.):

Any ideas on how I can get a "one measure to rule them all" solution that works no matter the context?