Pre-exposing 4x5 transparency film

In Ansel Adams book 'The Negative', he goes into a bit if detail regarding the advantages of pre-exposing both black and white and transparency film, primarily where increased shadow detail is neccessary.
He mentions the possible use of a color compensating filter sandwiched between two opaque sheets of plastic when pre-exposing color transparency film. The object would be two-fold: Open up the shadows a touch, and reduce the bluish cast of shadows caused by illumination from the sky.
What # color compensating filter(s) are we going to use for this, and can anyone be specific about the type, thickness, and opacity of the plexiglass he mentions?

I guess no one does this. I think I did once. While loading some exposed film onto the developing tank reels, the red heater lamp on my Jobo came on, and I realized then that I'd forgotten to cover it. I had some shots on that reel that should have had some deep blue shadows (Velvia in shade), but the shots came out rather nicely. I never went back to test my impromptu heater-lamp-post-exposure-burner, but I haven't been worrying about covering that lamp anymore.