Just another thought....trigger control, ie, not pulling the entire gun to the left while pulling the trigger is certainly one critical element. But...there is another. For a right handed shooter, the gun will recoil away from the palm of the hand, that's up and left...the stronger your grip the less the gun is affected in either direction. I too, use the thumb down along the frame and pinkie under grip as demonstrated above. For me, the thumb increases my lateral support, and the pinkie under helps with knuckle rap with heavy loads and is far quicker for thumbing the hammer back. The same grip, (thumb down) works with my 1911's as well and decreases the tendency for lateral dispersion.
Grip strength is critical...consistant from shot to shot and just short of the white knuckle, tremor stage. More like a strong handshake with a friend you haven't seen in a long while....strong but not hurting, if that makes sense. For part of my early pistol and revolver work, I squeezed a hand grip exerciser every day...as advised by Col Askins years ago... and like him, I found that it helped tame the .45 ACP.
I can move my groups several inches at 25 yds by the strength of my grip. For a two handed hold (which almost always increases the gripping strength unless I'm careful) my group will be an inch or two right of my one hand group.
I find little use for a sandbag as some have advised, finding that it proves nothing since it's not available when I'm on the range or in the field. By hook or crook, I've got to get the gun to shoot where I'm looking.
On one New Vaquero, in .45 LC, I played with the grips and adjusted the loads but still got gps 2" left at 25 yds. I opened the rear notch by filing on the right side then reblueing with cold blue. The combination of that sight adjustment and a tighter grip has me centered up now. I did have to remove .020" off the front sight height to get 250 gr cast lead bullets to point of aim.
On a bird's head .32 H&R, I tried to bend the front sight for the adjustment and promply broke it off. I re-soldered it at the proper angle and got a reasonable point of aim and impact, but the fact is that Ruger uses very hard, high temp solder and you're liable to knock the front sight off if you wack it with a ball peen and drift.
Just my thoughts and what's worked for me...your's can and probably will work out differently, but the principles are the same. Regards, Rodfac