osctrich is a simple graphic tool.
you need to load the spef from different tool and different corner. we can say
a-File>Import>SPEF
1-you load best & typical & worst spef generate from your PR (take care some place and route have different level of extractor, some simple to reduce the time processing, some for SI, need more time, but for SI fixing after PR is better to run a step of SI fixing violations)
2-you load best & typical & worst spef from signoff extractor
b-Correlate>Build Plot
you select one spef load for golden & one for target
c-Correlate>Draw Plot
will display the graph with the spef loaded and you can change the rc factor (left side) and recalculate...