How can we calculate the minimum sustainable number of the panda population?
Minimum viable population (MVP) can be predicted in two ways. 


Computer simulations.
Population viability analyses (PVA)


PVA is considered generally more common practice by ecologists and without a reference to the 1600 panda population you mention, my guess would be that this is what they did. PVAs work well for endangered species prediction. 

A typical example of PVA was carried out on an endangered species Fender's blue butterfly an endangered species (Schultz et al., 2003). However there is currently no scientific standard on how a PVA should be done.



Schultz, Cheryl B.; Hammond, Paul C. (October 2003). "Using Population Viability Analysis to Develop Recovery Criteria for Endangered Insects: Case Study of the Fender's Blue Butterfly". Conservation Biology 17 (5): 1372–1385.