We did this a while ago, may as well do it again. Let's see what homerun range Carlos Pena is looking at.

Balls in play over the last two years:

2007: 355

2008: 328

Amount of flyballs:

2007: 158

2008: 165

Amount of homeruns:

2007: 46

2008 :31

So put all of that together and we get that Pena puts about 47% of his balls into play in the air, and that of those balls into play, about 24% of them are homeruns. Assuming Pena puts about 340 balls into play, and 47% are flyballs, that gives you 160 flyballs. The good news is that the 10 homeruns he's hit can't be taken away.  Over these next ~120 flyballs, if Pena regresses to his career HR/FB% of 20.1%, he still winds up with more than 35 homeruns for the season.  That's only about four more than the projection systems were pegging him for and seems like a pretty realistic amount.  

Of course that could always fluctuate up/down along with his flyball rates, but this is a better way of projecting forward than saying, "Hey, he's got 100 plate appearances and 10 homeruns. If he gets 600 plate appearances he'll hit 60 homeruns!"