It seems to me like this project Flushing Volumes Calibration - V2 and new Gcode by AMT_MW - MakerWorld
and this thread Multiple poops with high flush volume please - #36 by AMT_MW
are exactly what you need.
I too don’t like the automatic flush calculation. Even with a 0,4mm nozzle, the calculated volumes are useless. Especially because it only looks at color but not material. If i print with support material I want to be absolutely sure that not the least carries over into the primary material and I couldn’t care less if it is green or purple.
I’m not surprised that it doesn’t make a difference of the nozzle size.