Are you positive about the .49% you expected? Take a look at the attached 2018.2 TWBX -- you can move your Rank_Percentile() Table Calculation to your data pane by dragging the Table Calc to the "Measures" area.
This will give you a new field named "Calculation 1" which you can edit, rename, and then use inside of a WINDOW_AVG table calculation. The difficult part is making sure the calculation has the correct scope -- I believe the attached .79 for BBB is correct, but let me know what you think.
I hope this helps!