In his book Using the View Camera Steve Simmonds outlines the method used by gordon Hutchings. He meters through the filter and then applies the appropriate factor for filter being used. In the case of a #25 it is two stops.
He expalins that Hutchings developed this system to make sure that a scene's shadow areas receive enough exposure as shadow areas are primarily illuminated by blue light. I have used this method for some time and it works for me.