How do we choose a frequency range of interest for flutter prediction?
You might want to try reviewing MIL-A-8870C  AIRPLANE STRENGTH AND RIGIDITY
VIBRATION, FLUTTER, AND DIVERGENCE.  The CRJs were certified to this standard.  Available here: http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-A/MIL-A-8870C_6746/  Just click the Download File button.