What I do, though it makes you look like a Chicken, is keep low...bend your legs...lock your upper frame, marker as well this will keep the top part of your body more stable and also stop the marker jumping around...
I usually put teh bottle into my chest but slightly lowwer than I would if standing.
If you keep lookinga t where you are firing or running to, you can adjust teh fall of shot as you go, but it helps to have an idea, of what angle you need to hold teh gun at off teh break, in order to reach your desired target.