I agree with stump. it doesnt take much. I usually draw my gear into my syringe and then just draw a little bit of gso in it as well. Usually if i have 2-2 1/2 cc of gear i draw 1/2cc of gso and everything is g2g. Just tip the syringe up and down to help mix it all up. GSO is a god sent for painful brands and painful gear.
I'm not a big fan of cuttinf gear. When you're mixing compounds nad trying to minimize pins per week it just adds volume to an already full syringe. Like right now I'm using 150mg of prop nad 100mg of npp eod. If i aded oil that would be 3cc of oil. I have cut some products when they were dosed high. And agree with above, I don't use any more than .5cc of gso.
Its going to depend on the gear, what solvents are used and what % BA. Most should be between 1-3% BA .5ml per 1ml should do the trick, just experiment with it trying to find the minimum ratio to address the pain.