Definition of unsaturated inductance of a multi-core transformer

[RHoschek]

After measuring the excitation curve of a multi-core transformer I have to define the unsaturated and saturated inductance of the transformer to enter them into a simulation.

While the definition for single-core transformers seems to be straight-forward, how would you define these two quantities for multi-core transformers?
Is there a standard procedure how to derive these coefficents from the measured curve?
How do YOU get them?

Thanks for any help or hint.
Robert