Kratom is an interesting plant that has different effects at different doses. Smaller doses cause stimulation and larger doses cause minor sedation. I prefer green vein over red or white, but only because its a perfect middle ground for me. I would advise starting at 3g and see how that affects you. I wouldnt go any higher than 6 or 7g because at that point, youre wasting your kratom. I have taken the same dosages of kratom twice a day every day for 2 years and have never needed to increase my dosage. I take 2.75g in the morning, and 3.75g in the evening. Just try a small amount, 2 or 3g, and figure it out from there. Your kratom will be potentiated by the buprenorphine, so start low.
Take it on an empty stomach, but make sure youre well hydrated. Also, dont redose for several hours because it does not have a good additive effect on itself.