the nominal seer is indicated in the model number if you know how to read it. otherwise you would need to consult some other literature. the system seer is determined by the combination of equipment and can be found through the AHRI directory. To my knowledge there is no universal rule taht sais the temp diff between coil and outdoor air should be X for a given seer rating. I would think it would have to depend on the manufaturer.
Standard ARI 210/240, Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment is at (80 degrees dry bulb, 67degrees wet bulb) inside air and 82degrees outside air for a single speed compressor and indoor fan.