Right on, I am guessing you already understand minute of angle and basic wind and gravity. ?
I would say go here
http://www.iowahighpower.com/hplinks/databook.htm and download a some data book sheets. You can use them to start recording your dope. I would also recomend buying one of these
http://www.ambientweather.com/amhp816a.html it's a cheap get you started tool that will make collecting your data 10 times faster and easy.
What you should do is fire 5 round shot groups at each disance and then record the bullet drop and the wind push. So if you are zeroed at 100 and shooting at a target at 300, you will record hom much your bullet droped and if there was any wind, the value of that wind and how far it pushed your bullet. Of course fill in the temperature and the other data. What this will allow you to do is build a record of conditions, so that when you are in those conditions again you will know what adjustments to use.
If you can shoot sub-moa at 100 then you can do it at 400, it's just a matter of knowing the effects of wind and gravity on your bullet/gun combo and being able to adjust for those effects.
As for the position shooting, you will need to do a lot of dry practice of building a solid shooting position. Follow that up with shooting at 100 where the wind and gravity are not effecting your shot, so that you can get the best results prior to adding the other effects.
To be honest I do not know of many people who can hold sub-moa groups in the standing or kneeling positions, is it possible (yes) but I would not set that as a goal. I would look for something that you can build up to and then improve on. Say like 10 shots on a bulls eye at distance with a score of 75, and then try and push that score to 80 and then 85 and so on.
Dry drills/ dryfire is going to be the key to position shooting, You have to build the muscles to support the rifle in each position prior to managing the recoil. So I would say practice 5 minutes in each position doing dryfire, once you get to a point where the fatigue is not as bad make it 10 minutes and then 15 minutes. The longer you can hold a position the better...
If you own a video camera, video your self shooting. This will allow you to maintain a record of the shooting session and allow you to see for your self what you are doing right and wrong. Being able to see that you changed something while shooting a group, that may have caused a bad shot or being able to see what your doing to achive good shots is a big plus.
For wind and gravity read this, it's a pretty good post.
http://www.shadowspear.com/vb/showthread.php?9341-External-Ballistics&p=123292#post123292
For marksmanship refresher
http://www.shadowspear.com/vb/showthread.php?13956-Aka-Fundamentals-of-Marksmanship-with-a-scoped-rifle.&p=195268#post195268
Hope this helps!