The fellows at minorleaguesplits.com put up a minor league equivalency calculator on the site so one can see how a player's minor league performance translates into major league performance factoring in level of competition and ballpark effects.
You can also run it in reverse and see what the equivalent performance by a major leaguer would be if he were back in the farm system.
For fun I prorated Adam Dunn's Washington Nationals numbers over a full season and then stuck him in Kane County, the Oakland Athletics affiliate in the Midwest League (and wouldn't Dunn just fit in great with the Athletics?).
Dunn's full season numbers equate to a .553 batting average, .709 on-base percentage and 1.272 slugging percentage in Kane County. The raw numbers:
444 AB, 177 R, 246 H, 54 2B, 0 3B, 89 HR, 239 RBI, 230 BB, 154 K.
Wouldn't that be fun to watch Dunn for a full season at Class A?
Used properly, though, the calculator is a nice tool. I have been dismayed that the Royals have not promoted native Hawaiian Kila Ka'aihue this season despite his .400 OBP. Plugging in his numbers from Omaha, you see that his low batting average translates into an almost unusable batting average at the major league level, despite his ability to draw walks. Then again, it's the Royals, what could it hurt?