There are no hard and fast rules, James, as I see it. There are suggestions of what can work for you, or we can make suggestions on what works for us.
Some clubs will not allow an elbow to touch the ribs or hip - to them, "off hand" actually means "off the hands"- it is a non-supported shooting postion.
We allow any style position you can think of, as long it's safe and are your standing and holding the gun in your hands. LB favours his hands close together, like Roger. Some of us more aged and/or feeble types
, cannot shoot that way.
Best 100 yard offhand group I ever shot was with a 22 pound unlimited BR rifle in .308. Yeah- I was only 22yrs. old. 98/100 on a postion target. Beat my best kneeling score by a single point. I used a dropping system as there was no other way to make the shots all 10 of them. When the cross hairs almost touched the top of the 10 ring, I squeezed off each shot. My first 2 were sqeezed off too late, as I touched the X ring and dropped out to low 9's. I rasied the time of firing and received 8 tens in succession. I think 5 of them were X's.
With a fast lock time, this sort of timed shooting works. I also found a caplock worked almost the same, but more time was needed, but timing the shot worked- from the side was my preference then. Now, it's raising from the bottom for apreference with cap guns, but I will take whatever is happening.
For me, with a flintlock, this sort of timing does not work - at all. What happens if the ignition is a bit faster or slower or a hangfire could even have you 6" off the target, high, low or to the side depending on your angle of swing. How far does the gun actually move during the ignition sequence? for the lfintlock, I have to try to hold on centre, through the shot. So far, it seems to work fairly well. With the weight training, I am holding better.
Please be aware that this in no way is a disparaging post, nor is it saying your post is incorrect - it is merely meant to bring to light, some of the idiocyncracies we have in offhand shooting of ML's in comparrison to modern rifles.
The variable timing adversly effects ball impact.
The only way to find out what works, is to get out and experiment- actually burn some powder. You cannot learn to shoot without shooting, just as you cannot to build, without building.