Assura QRC extraction

Hi, I have a question about Assura QRC extraction.

When I have multiple ground nodes, which nodes do I need to set as a ‘Ref Node’ in the ‘Extraction’ tap of QRC parasitic extraction run form? Otherwise, do I need to just set the ‘Ref Node’ to the backside metal such as ‘wafer!’?

The reason I started to have a question was that the QRC simulation results varied depending on which ground node I set as a reference node.

Thanks,

Jinsung