Hi,
I will refer you to the section III-C of this paper: "
Appearance-Based Loop Closure Detection for Online Large-Scale and Long-Term Operation".
Some values come from this paper: "
Fast and incremental method for loop-closure detection using bags of visual words".
FAB-MAP uses 0.9 too for the new place prior:
"the prior that a new topological link leads to a new place was 0.9"
For the PredictionLC, this is a gaussian of sigma=1.6 (as described in the first paper above). The choice of 16 is dependent on the neighborhood used to retrieve the locations from the Long-Term Memory. 16 was satisfying for every datasets tested: enough large to retrieve more future nodes in case the environment is dynamically changed, and not too large to avoid filling the WM with only neighbor localizations.
cheers