EHV busbar protection slope

[powerpi]

Hello

I have a recurring question about the settings of a low impdance busbar Protection systems. We are using mainly Siemens and ABB products and leaving the default slope (k) settings of e.g. 80 % (k=0.8). However, it constantly puzzles me on how to calculate the proper slope regarding the switchgear short circuit conditions and all the used CT's. We use mainly 5P20, 20 VA, 1000/1 A for BBP, Iscc is never higher than 40 kA. Does anyone has any systematic approach on how to calculate the proper slope (k) of the BBP function in regard to the actual circumstances in the switchgear?

Thanks

Blaž