For 915Mhz center frequency, the default numbers used in the RFM69 library translate to:
Bitrate: 55,556
FDev: 49,988
Rx Bandwidth: 125,000
AFC Bandwidth: 100,000
For comparison, plugging these numbers into the calculator shows that the same FDev would also support a Bitrate of 200,000. Alternatively, for a bitrate of 55,556, FDev could be as low as 13,977.
Although the calculator does put-up a red-flag if FDev is too low for a given Bitrate, or if the Bitrate is too high for a given FDev, it doesn't red-flag either of the bandwidths if they are incongruent with Fdev, nor does it red-flag if the Rx and AFC bandwidths are incongruous with each other. That makes the Semtech calculator less useful than if it were to check whether all the numbers satisfy all the constraints stated in the datasheet, as I was hoping that it might.
So, all in all, the calculator is perhaps partially useful. For instance, it does appear to compute a DCC frequency for both the Rx and AFC bandwidths. Also, the calculator will automatically translate whatever figures you give it into the corresponding register values, which might be a useful cross-check.