jgauthier,
80% is a good number to use across the board. I will say, there is a trick you can use to ensure you are always getting the maximum available bandwidth. If you simply set the bandwidth equal to the sample rate (100%), then after you configure the receiver you can call the bbQueryStreamInfo function, and it will report the bandwidth that was actually used by the API. It will internally clamp to the upper bound for each given decimation.
The maximum bandwidth for any given decimation was calculated by looking at the decimation filters available at each decimation stage and making a decision about what cutoff frequency is going to give us the performance we need at that given rate.
Regards,
Andrew