Playback distorted depending on aspect ratio on Adobe + FCPX

Hello,
It has been asked here already, but no answered, so I'm asking again.
Using Decklink Mini Monitor 4K with After Effects, Premiere or FCPX,
I can't have a proper playback aspect ratio when I'm working on a composition/sequence which is not the same resolution than my preview monitor.
— Working on a 1920x1080 pixels sequence on FCPX > 1920x1200 monitor, the picture will be stretch vertically to fill all the monitor.
— With a 1000x1000 composition on AE > 1920x1200 monitor, stretch horizontally, impossible to work with.
Where is the setting on the Blackmagic preference pane where you can ask the card to create a letterboxed to maintain the proper aspect ratio?
Thanks!
It has been asked here already, but no answered, so I'm asking again.
Using Decklink Mini Monitor 4K with After Effects, Premiere or FCPX,
I can't have a proper playback aspect ratio when I'm working on a composition/sequence which is not the same resolution than my preview monitor.
— Working on a 1920x1080 pixels sequence on FCPX > 1920x1200 monitor, the picture will be stretch vertically to fill all the monitor.
— With a 1000x1000 composition on AE > 1920x1200 monitor, stretch horizontally, impossible to work with.
Where is the setting on the Blackmagic preference pane where you can ask the card to create a letterboxed to maintain the proper aspect ratio?
Thanks!