Offhand shooting of any firearm depends on establishing your natural point of aim before each and every shot you fire. Once you have established it, you DON'T want to move your feet, so when you check each time before you fire, you should still be on your natural point of aim. Now I realize some BP shooters prefer to load at the bench and then move into the shooting position to shoot. If you do that, you MUST re-establish your natural point before every shot. Also, do this when dry firing with a wood flint at home.
Dry fire at least 10 times for each live round fired. This will strengthen your upper body and reduce your "wobble area" when shooting offhand. This will also help teach you not to be concerned about the movement of the flint or of it setting the priming off. This will also show you if you are flinching and it will help you learn how to call your shot.
When mounting your rifle/gun, always try to get the buttplate in your shoulder the exact same way each time. Same thing for your cheek weld (where you place your cheek on the stock) and how you hold the forearm of your rifle/gun with your non shooting hand. Uniformity in these areas will give you much better scores.
Take two or three somewhat deep breaths and hold the last breath as you smoothly and consistently pull the trigger. While doing that, I can't stress enough that you MUST maintain as perfect of sight alignment as possible, even though your wobble area or natural movement will not allow you to remain perfectly still and center on the target. You concentrate on perfect sight alignment even after the shot goes off and until "the bullet has a chance to go through the target." This will ensure correct follow through.
Now IF the shot doesn't go off before you run out of breath, then relax the pressure on the trigger and take the rifle/gun out of your shoulder and relax/breathe for a little while before trying again. If you hurry the shot because you are running out of breath, I can guarantee the shot will be a flyer.
As far as knowing when the shot is about to go off, I have found over the years that for MOST people, it is best for it to be a surprise. Please understand I've been doing trigger jobs for National and International Competition in BP and Modern shooting since 1973. I first joined THE Marine Corps Rifle Team as the Junior Armorer in 1975 and that's where I really learned "the Arte and Mysterie" of adjusting trigger pulls to the preferences of many of the very best shooters in the nation and world. Now, I'm not saying a minority of shooters don't do better when they know when a shot is going to go off, but they are far more the exception rather than the rule.
Finally and I don't care how great the shooter is, EVERYONE is going to screw up and occasionally shoot a bad shot. In National and International Competition, many shooters can win a match, but it is usually the one who makes the least mistakes that day who wins. So when you shoot a bad shot, forgive yourself and then go right back to trying to do everything as perfectly uniform as possible.
Gus