How do you measure how much RSO to add to cream/lotion?

My local med dispensary has RSO that I wanna make a topical cream with for my spondylitis pain. My goal here is to make it worth 400-500mg strength. I am a newb and still don't know wtf I'm doing 😂

For context, the strain I'm looking at is a 1:1 ratio: 37.32% THC (500mg) and 38.44% CBD (500mg). It is in a 1g syringe. As I said before, I want the strength to be 400-500mg and make 8oz worth. Help?