Adox CMS 20 Reciprocity

I have tried a few rolls of Adox CMS 20, and I am intrigued by its sharpness and lack of grain. But at ISO 20, it's easy to get into longish exposure times in anything other than bright sun. I haven't been able to find any published reciprocity failure guidance for this film, so I have been using generic guess exposure increases at one second and slower with mixed results. Does anyone have reciprocity data specifically for this film, or at least a reasonable rule of thumb?