If you need to shoot one-handed while seated, there is a lot of carry-over and correlation between the things you don’t want to do when shooting one-handed while standing. Rob Pincus shows two basic tendencies he has observed in relation to body positioning.
The first thing he has noted from students doing shooting drills is that when standing, people want to lunge forward with their strong-side foot and then blade themselves out. When seated, that’s obviously going to be harder to do as the hips will be relatively stable due to the seated position. The shooter can rotate his upper body, which isn’t as bad as when standing and able to rotate the whole body, but it’s still not optimal.
Come back in and set the shoulder so you can make sure the gun is properly aligned with your line of sight. Note that Rob’s shoulder is still in front of his hip. He’s not pulling back to a position that wouldn’t give him a supported platform. He has his weight forward but is not lunging forward dramatically with the strong side.
Another tendency people have if they are seated in a chair that has arms is to plant against the chair arm on the support side. This is fine if you’re using it to stabilize yourself but not great if you’re using it as a base to twist, push and lunge forward. Keep the torso square so your gun alignment and recoil management are as consistent as possible with your standing position.
Most of your handgun training will be done in a standing position. When seated, you want to maintain the same strong body position as you use when standing to ensure proper aiming. Keep the torso square and drive the gun straight out. The support hand can touch the chair, or may be busy doing something else, or may be injured and is the reason you are shooting one-handed in the first place.