I agree it seems to be a gray area, or a case-by-case area.
If the child pays $360 as "fair share", what is the other $500 spent on?
If the SSA determined that $360 is support "she pays" and the rest is not, maybe that is good enough for the IRS. Then you only need to show that his actual living expenses are more than $721.
You may still want to go through the entire pub 501 worksheet, it may make this all a moot point.
