Length of pull for sitting is usually noticably longer than for standing. Not many make adjustments to the length of pull, and you have to remember the scope limits that as well. So there's a compromise often made. You have to figure where you're having an issue, with the majority of sitters, or the minority of standers.
For new shooters I generally advise going for the low hanging fruit and setting up for sitting, and seeing what they can pick out of the positionals. As one progresses, you can change the balance if you need to.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep