question about Ferrite Materials and Ports

Ansoft HFSS-Material Manager,as follows: 
 
Ferrite Materials and Ports 
When designing a problem containing ferrite materials and ports, do not arrange the port so that it touches the ferrite material. If you must place a port on a ferrite material, separate the two with a dielectric with a relative permittivity equal to the relative permittivity of the ferrite. 
 
The question is: how to know the size of the dielectric? 

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