If you are interested in the threshhold that FEMA uses for its presidential declaration, a new GAO report questions the formula FEMA uses. Federal Disaster Assistance; improved criteria needed to assess a jurisdictions’ capability to respond and recovery on its own. GAO provides both an executive summary and the full text report (89 pp). The GAO is critical of the one criterion FEMA uses, namely the per capita damage indicator.
I highly recommend this report because it contains a lot of interesting information about declarations and the declaration process.
See Eric Holderman’s blog on the same topic. Eric has the practitioner community perspective.
Recovery Diva 
Thanks for bringing this to my attention. Valuable report and blog.