As others have said, everybody is different and there's no universal answer. I can only tell you what works for me. For a few months after being diagnosed, I kept a food journal and tested myself (finger-prick test) before, and two hours, after each meal. That gave me enough data so that I could see exactly how much the readings would go up and down for different amounts of carbs. Then I could work back to see how much I should limit myself to, to keep the levels below any chosen target.
For me, it worked out that 120g of total carbs a day, spread evenly over three meals, would keep me below 7.6 which I decided was a safe level to aim for. And it worked, after three months my HbA1c had gone down from 50 to 43. I'm hoping to get it down a couple more notches when I'm next tested.
I'm not saying that 120g is a useful number for anyone else to aim for; you have to find your own number. But correlating levels with measured food intake is really the only way to work it out.
Hope this helps.