As you are prediabetic, I would suggest eating the amount that satisfies you. Eat so your hunger is sated but you don't feel stuffed. Protein does turn to glucose through a process called gluconeogenesis. However, it's a much slower process than with carbohydrate. There is less strain on your pancreas due to this. It can produce insulin more slowly rather than being hit with a massive quota for a sugar rush.
In layman's terms, if you find you are gaining weight, you are probably eating too much protein. If your weight is staying constant (or falling if you are looking to lose weight) then you are probably eating about the right amount. Also, if in any doubt you could get hold of a blood testing meter if you have not already. Test your blood sugars a couple of hours after eating breakfast, lunch and dinner. If they stay within a normal range then you are doing good work.