filter factor for Leitz GGr green filter

Since they don't make green Series VIII filters new anymore, I found a used Leitz GGr green filter.

Does anyone know if I should expose 1 or 1.5 more stops as a general starting point?

The filter factor for an X1 (#11) filter is 4X or 2 stops. A green B (#58) filter has a filter factor of 5-6X or 2 1/3 to 2 1/2 stops. This is with Pan film. If it's yellow green I would start at 4X or 2 stops, maybe even on the other greens too.