Ratios for CBD to THC typically come in a variety of ratios. Some of the most common are: 1:1, 4:1, 8:1, 18:1, 20:1 nd a very high ration of 28:1. People will often talk about the entourage effect when discussing why to use CBD with THC. You can read about that here:
Why to choose a higher ratio of THC with CBD is a personal choice and can also be a choice based of the overall effect you want to create. A 1:1 may be slightly psychoactive for some and may also provide more instantaneous pain relief. I treat my migraines with a high ratio of CBD to THC using a 20:1 several times a day. I choose this as it is both preventative for me and not psychoactive.
Both CBD and THC are considered to have their own therapeutic properties. I might suggest reading this article as well:
I am not a doctor and this represents my opinion only. I hope this helps!