you have to define it in the parameters, I do this before I run stamps(1,1)
You can select areference areaby setting parametersreflonandreflat(for a rectangulararea) orrefcentrelonlatandrefradius(for a circular area). All plots will then be refer-enced to the mean value for this area. If not set, the reference value is the mean value for the wholearea.
have a look here https://homepages.see.leeds.ac.uk/~earahoo/stamps/StaMPS_Manual_v4.1b1.pdf and search for reference