"Start low and go slow." Start with a low dose and go slow in taking more until the effect is fully realized.
I would recommend starting with a 1:1 CBD/THC, which is the standard for pain relief. This means, there are equal parts of CBD and THC in each dose. Next comes determining the milligrams in each dose. I would start with 2mg of 1:1 THC/CBD. See how you feel, did you experience any pain relief? If you did not feel sufficient pain relief, then take 4mg. You should always consult with the pharmacist at your medical marijuana dispensary, as they are great at helping you find your optimal dose.