"CBD:THC ratio of >10:1 for zero stage cancer, with no side affects, could I up the dose to 20:1 ?"
Yes, you can take as much CBD as you want or feel you need without any risk of injury or impairment. Worst thing I have seen from taking too much CBD is temporary fatigue. I usually recommend 90mg of CBD per day that has been very effective for a broad range of cancers. But I do add another cannabinoid that is widely available online. Many stage 4 have gone into remission with this protocol.
What type of cancer are you treating and what is your goal? Palliative/symptom relief or trying to fight the cancer? We have seen best results when the acidic cannabinoids (CBDA, THCA) are added in as well as CBD:THC. The ratio and quantity depends on what type of cancer and your goal.