Trouble using SELECTEDVALUE in already filtered values.

This might be a bit long, so I appologise in advance.

I'm trying to set up a What-if parameter scenario that will simulate changes in market share.  This implies that an increase in one item of a group will have a proportional decrease in the rest of the group.  The example is airline routes.  Here is the table relationship structure.

The link for both the outermost tables with the airports table is the departure airport code (ICAO).  The market to be analized is determined by filtering on "City" from the airport table, and "ades" (destination) from the fact table on the left.  This will give a list of all the airports from that city that have flights to that destination.

Now, using a slicer based upon the "Departure Airports" table, I want to be able to select which departure airport will be the driver of the change in market share, and have that change affect the other routes in the filtered group.

Here is an example in Excel for a London market where the flights from Heathrow increase 10%.  The decreases in the other departure airports is proportional to their percentage of the flights from that smaller group that does not include the selected departure airport.  This way the total flights from the market stays the same.

When I try to set it up in PowerBI, I have trouble capturing the number of flights from the selected Departure Airport as a variable.   Here are the slicers and table organized in the same manner as the relationship image to be able to show which tables the slicers are working on.

And here is the code to try to calculate "Sum of other origin flights" (right side column in the table).

Total of other orign flights =
VAR SelectedOriginFlights =
    CALCULATE (
        FIRSTNONBLANK ( 'pbi GESTAR_adep_ades_distance'[flights], 1 ),
        FILTER (
            'pbi GESTAR_adep_ades_distance',
            'pbi GESTAR_adep_ades_distance'[adep]
                = SELECTEDVALUE ( 'Departure Airports Selector'[DepartureAirport] )
        )
    )
VAR AllFlights =
    CALCULATE (
        SUM ( 'pbi GESTAR_adep_ades_distance'[flights] ),
        ALLSELECTED ( 'pbi GESTAR_adep_ades_distance'[adep] )
    )
VAR OtherOriginFlights = AllFlights - SelectedOriginFlights
RETURN
    OtherOriginFlights

Is this the right approachand I'm just missing something?  Is it even possible?