In developing your positions, there are two critical issues. The first is RELAX. Relax every muscle you can without falling on your face. Skeletal support alone is what you're striving for. Second is NPA. Natural Point of Aim is where the sights are looking once you've built your perfect, relaxed position. Develop your position with your eyes closed so you won't compromise it by trying to move your sights to the bull. Move your feet (offhand), backside (sitting), or your hips in Prone to orient your built position to the center of the target.
A coat will aid your Offhand tremendously and keep the sling from cutting into your bicep. A glove or mitt is all but mandatory.
Shooting Service Rifle with a post, the only thing your eye should see is the square top and edges of the post. If necessary, find glasses (readers) that allow your relaxed (there's that word again) focus to be exactly on the post. The target doesn't matter as much as you might think. It should be slightly blurry. That said, learn both a six o'clock hold and a center hold since weather and light conditions sometimes dictate which works better. BTW, the difference in elevation is about 3 MOA, regardless of yardline.
Practice Prone at reduced ranges until you're shooting near-cleans BEFORE moving back and introducing the meteorological calculations.
Here are some printable, reduced course targets - http://gamecalls.net/free_stuff/52_free_targets.html
Best "textbook" on Position Rifle Shooter evah - 2009 Ways of the Rifle from MEC in Germany.
Good luck! Keep at it - Highpower, Smallbore, and Bullseye Pistol are HARD. Because they are hard, they're very rewarding.
And .... relax.
Mark
Distinguished Rifle #1228
Distinguished Pistol #1500
NRA High Master