TREY526,
What limitations do you have? As Mike asked, knowing what code they rated you under would tell a lot. But, I also suspect that there may be other rating available. For example, if they only rated your shoulder, but you in fact have problems with breathing, or nerve damage, or psychiatric disabilities, these may add up to an additional basis for rating that could get you to 30%. If you do have additional injuries, but they were not properly addressed, you may want to look at gathering additional evidence (this may be medical evidence, or perhaps getting an updated Commander's letter/Non-medical Assessment). Have you spoken with your JAG? Attorneys are given access to the worksheet/rating card that the IPEB uses to make notes on their reasoning for findings. Knowing what is in that can be very helpful.
Do you think that you might want to continue to serve in a Permanent Limited Duty Status? An interesting (but as of yet untested) legal issue is how the changes in DoDI 1332.38 (as directed by the DoD Policy Memo Implementing 2008 NDAA) regarding continuation in service after a fit finding impact the services processing of these cases. I do not know if your disabilities would allow for this (or if this is something that would interest you). However, I mention it as it may be worth exploring.
I hope all goes well for you at your Formal hearing.