Last week, Jason Hanselman wrote about properly regressing projected platoon splits over at The Rays Way. If you haven't already, you should give it a read, as it explains the concept pretty well. This isn't anything new or groundbreaking (The Book, from which the methodology comes, was published in 2007), but it is rare. We sabre-inclined baseball fans too infrequently take the time to actually do the easy calculations, and instead just cite career splits.
Another Cubs Blog used to have a regression tool, but the link doesn't seem to work anymore, so I went ahead and made the spreadsheet myself. I was referencing their post by berselius as I made it. I shared my spreadsheet with the other writers here, but the Jeff Keppinger signing makes it pretty clear that everyone should have regression at their fingertips.
Feel free to download and use my Regressed Platoon Splits Calculator (version two). The white columns are objective stats. The orange column is for an overall wOBA projection. The green columns are the career and regressed splits (as a percentage), and the blue columns are the projected wOBAs against each handedness.
Edit: After a rereading of that chapter from The Book and some input from Sandy Kazmir and Sky Kalkman, I've added added switch hitters to the tool.