I think TheRock's way works, but here's another way to think about it:
First find what the average BA of the entire draft pool would be: Take all 168 hitters drafted (or however many your league drafts) and get the league average BA. Let's say it ends up at .270.
The basic formula Tango suggests is:
H - (AB * league_average)
So if A-Rod gets 159 H in 549 AB:
159 - (541 * .270) = 159 - 146 = 13
Here's what that formula is doing: Let's suppose that a league average player (the .270 hitter) gets the same number of AB as A-Rod (541). In that situation, how many hits would he have? He would get 146 hits (541 * .270).
Now A-Rod is projected to get 159 hits. When we subtract the pro-rated hits of the average player (146), we find that A-Rod is projected to get 13 more hits than the league average. That 13 is what Tango uses for xH.
EDIT: I can't subtract.