a good off hand stance should be a position that is very relaxed and certain ways to hold your position and the rifle will be basicly the same for most folks with a few mods depending on what works for you and fits your body.
my off hand stance is basicly the same but does change slightly for the conditions which effect it and that is mainly caused by how windy it is.
I will put it simply before going into any detail and that you know you position is right when you can hold the rifle very still right on the x ring or at the 6'olock hold and it does not move and you can break the shot and know it went straight down the middle.
on a calm day a very good off hand person can and should be able to do this on every shot. you have to say to your self do not settle for anything thats not a x ring wide wobble and acept 10 wide at the worst and if your wobble area is worse than the 10 ring you need to correct the position untill it calms down to a x ring or at least a 10 rign hold.
now back to your question on the hips and feet position.
the common way for high power shooters is to lean back a little to be able to get your front elbow more tucked into your lower rib cage area and to turn and pre load your back so you may have a position that might not go any futher one way when you are trying to shoot . this works great and even some big names use this to a degree and is a great way to start to at least get you close till you under stand everything you need to know.
ok here's a few items that may help but its advanced and you may need someone to watch you to correct bad things or things that you may not know you are doing.
#1when I get to the line and have my stool and scope set up where I know I will not need to move my feet till I'am done shooting.
#2 rifle in hand I will align my body and feet at 90degree's to the target (like you said) and feet shoulder width apart to start.
#3 I will look my rifle butt into my arm-shoulder to make sure its the same each time and then bring the rifle down into the off hand position.
#4 the rifle should be as close too and over your body as possible and what feels comfy as doing this puts the weight of the rifle in close and it makes it more controllible.
#5 my foward arm that is on the hand gaurd comes down and rest on my side in against my rib cage and slightly against my hips and all the tension is released and I relax my foward my and hand to the point of it just being their and supporting the rifle as my foward are position is such that all the weight of the front of the rifle is on my bone structure that runs down threw my arm and is at no time ever supported by any muscle.
#6 now when in this position instead of turning to the left to preload your body I twist my upper body only inside of my jacket to the right (right hand shooter) with out moving the rifle or your elbow off your ribs keeping the jacket in the slight twisted position and then moving your upper body back to its normal 90 degree position inside of the jacket and that locks your upper body position without any tension that can change.
#7 know with the rifle coming to your head and not your head to the rifle you should have a almost erect head position and not leaning over into it. look down the sights and get your alignment and after you have the sights aligned in the front center your rear sight as you aply check pressure and after you do this do not move your check off the rifle till after the shot as that will keep you aways aligned correctly and now where ever the front sight goes that is you impact and yes its going exactly where it should if you followed that head position.
#8 now its time to check your balance with the rifle in the fireing position start to get your breathing slowed down and relax ( I move a little like a wiggle to settle everything in) and pretend you are going to try and break the shot then close your eyes and feel if you are swaying foward or to the sides. this is the time to correct that. if you are swaying left or right take your front foot and move it ever so slightly left or right and to fine tune it matbe just your toe position. if you are swaying foward and back move you front foot ever so slightly foward or back. do this till your sway stops .
#9 now you are almost ready to shoot but you need the most important thing your NPA (natual point of aim) and to do this after you have done everything else close your eyes again and breath and relax and then open your eyes and see where your sights are. if they are not exactly on the x ring or where you hold it then you need to change it till its very close. if you open you eyes on the sights either left or right move your lower body position ever so slightly antill it right on and if its low or high move your foward foot in to lower your sight or foward to raise it. repeat everything till its correct as you have 3 minutes to prepare and it should not take more than a minute to do everything several times to fine tune it.
#10 I will not get into hand positions or how to adjust your hold or stance in the wind at this time but do remember one other thing and that is your npa will change on most every shot and that is because of your body getting looser and other things.
you might try and tretch before a match to loosen up and a 10 shot string might go like this. shot 1 perfect , shot 2 now your x hold is a nice 10 hold ok, shot 3 now you are more relaxed (I hope) but you start to see that you have to move your rifle a little to the left or right to get into the black STOP RIGHT THERE your npa has changed even when you know you have not moved your feet. rebuild your position and start over with that x again and when those sights start to dance its telling you to start all over.
sounds simple does it not? but you break one of those rules and then a few more and thats when a x can become a 6 or 7 or worse and you find that when you think about that one shot that went bad you might of made 3 or 4 mistakes on that one shot.