Whenever i had a red card i lost about 5% of possession.. ofc it depends, if i get a half time it would be 2% or so..
What i think about this is.. All players in yellow( MC-ML-MR-DMC-DMR-DML-AML-AMR-AMC ) should afect the possesion in the same way, not just the MC.. so the more players in yellow you got on the pitch, the more possession you get.
Maybe 3MC formations get boosted cause they've got AML/AMR or DML/DMR to play next to them ? uh idk.
I think that 3DC-DML/R 2MC-AML/R ST would give the maximum possesion boost as it has 6 players in mid positions.