Given the size of your data, this is likely a non-starter, but one method is to try a self-join
such that every flight in a Line knows of every other flight in that Line.
That is, joining the dataset to itself based on Line#.
Then the first version of [Flt] is used as the filter
and the second version of [Flt] is used to populate the Gantt.
Because of potential duplications, you may need to use
aggregations like MIN to get back to the true measure values.
Please see the attached in the Forum thread.
Thank you Gantela for the idea. But, as you predicted, my data size did not make this a viable option. I did, however find a solution by using a custom query to pulls the parameter value to pull the data from the source. Here is a copy of the query for those who are having similar issues in the future.
The End User simply puts in the flight number into the parameter box and (after a few seconds to run the query) the line of flight that encompasses that flight number is displayed.
Glad you found the solution.
All the best.