What is the best CBD/THC ratio for SCA?

By SCA I assume you mean the neurological disease that goes by that acronym. Please realize there is virtually no clinical literature describing a correct cannabinoid ratio for this, or any disease, and the recommendations offered should not be confused with treatments that have been validated using clinical science. We do know that the endocannabinoid system has 'neuroprotective' functions which makes its stimulation appear a useful thing to try in your case. We also know CBD is remarkably non-toxic, making its regular use not harmful in any way yet detailed by science. I would advise whole plant preparations to take advantage of the 'entourage effect' and including generous amounts of CBD, greater than 1:1. As to dose, I would say "as tolerated," meaning starting with low doses and gradually increasing the amount taken, staying below the amount causing any unwanted effects. A realistic goal would be comfort and reduction of disease progression, not regaining function. I hope you find this helpful.