I uploaded an excel spreadsheet on Github with a table of achievable air transmission rates for the RFM12B radios, including code instructions for my RFM12B library.
You can tweak these to achieve better range or better penetration. It’s hard for me to provide specifics since in my case the default 38kbps works well. This guy on the jeelabs forum achieved much better reception by lowering the kbps to ~9kbps in his environment.
If you use this and get interesting results, I’d love to hear about it.