What "rule of thumb" to use for mapping -3dB frequency to ideal response for 2nd order Sallen-Key?
There's many ways to answer that, here are some to throw out ideas:

1) since it is a second order filter with 2X slope per octave then you can argue that it should be sqrt(10) X.

2) typically though that measurement might also take into account the ripple and constrain it so that a fixed factor may not be apropos.  In your case of critically damped this doesn't hold, but in a more general sense perhaps a simple factor isn't the right criteria.

3) Since I work with S&amp;H system and switched cap we don't use a 10X but how many time periods until settled to within the error budget of the system (so it's variable depending upon what is being designed).