On many models carrying a rifle, it's held across the chest in a fairly predictable position, and the torso twist applied in the earlier step will inform where the muzzle of the weapon ends up pointing. Where possible I try to lean towards torso/weapon positions such that the muzzle falls within the angle described by the feet in the previous step (#1 and 2 above), as that approximates a braced firing position. Models whose torso is canted around to the left will naturally have the muzzle of the weapon fall outside the angle of the feet (#3 above), but that's okay as we'll discuss in a moment. In order to get a model to point a rifle held across the chest all the way around to the right, the model's torso would have had to been twisted far beyond the 45 degree mark and would look anatomically inaccurate (so generally I don't ever end up with any built models that would look that way), but there are character models brandishing one-handed weapons that may 'break the rules'. In those cases, I try not to have the weapon pointing at a more than 90 degree angle from the line drawn through the opposing foot (#4), as that would mean the model would essentially be aiming behind it which can look rather awkward.
The last step is the head, positioning of which is very important. The main rule I follow here is that the angle of the feet is more important than the position of the weapon when deciding which way to point the head. Where the muzzle of the weapon weapon lies within the angle of the feet, I tend to have the model's head sighting down the weapon, as that results in a natural-looking firing position (#1 and 2 above). When the muzzle of the weapon points outside the angle of the feet, the head should STILL face somewhere within the angle of the feet (#3 above), as that results in a model that appears to be advancing with his weapon held at the ready prior to assuming a firing stance. The exception applies for characters with one-handed weapons, and in those cases the head may be canted a little further around to sight down the outstretched barrel, but I stick with the 90 degree angle from the opposing foot rule here as well.
Bashaws Lon and Shah here are examples of the one-hander rules. In these cases I dry-fit the arms onto the torsos first before gluing the torsos to the legs, to ensure that the weapons positioning wouldn't violate 'the rules'. Lon's sword and pistol ended up lining up nicely with his feet, and as his sword is the obvious direction of interest, his head was aligned to sight down the blade. Shah on the other hand sights down the barrel of his pistol which is just shy of a 90 degree angle from the torso. Canting the torso around such that the weapon falls within the angle of the advancing feet meant that it would have violated the 45 degree twist rule, so that means instead his head must point outside the angle of the feet in order to align with the pistol, but by keeping it to within the 90 degree angle from his right foot it still looks like a natural pose as no single element of the body is twisted around farther than would be natural.
Not sure if that explanation makes a lot of sense, but happy to answer any questions about the method!