As I don't shoot the national circuit my contribution isn't based on observations of others. When I did the club instructor training course I was shown the recommended way of using a bag for kneelers and that is how I teach all my newbies at our club.
Face your body about 60 to 70 degrees across the line of the shot. Place the bag on the floor just in front of the toe of your back foot. Place the other foot forward and parallel to the line of the shot. Kneel down on the bag so that your knee is on the far edge and the bag is under your shin.
Keep your back foot vertical. The base of your spine is allowed to touch the heel of your back foot. Place your front foot so that the shin is vertical or with your foot slightly forward of that position, to throw your weight backwards. Do not bring your front foot nearer than the vertical shin position. (Except for elevated shots when there may be no alternative).
Place your forearm along the line of the front thigh - if you can - and make sure that the wrist is unsupported and takes the weight of the rifle.
This is not a very good quality photo as it's enlarged from a wider shot but it demonstrates the above. Can you tell me if my position is legal? (apart from being left handed of course...)