EBM is more watery, hence you have to give more usually. But then, there are certain times where there will be milk spurts then after that, it will be back to normal. Your baby will tell you if he is full or not. I will not be so concerned about overfeeding.
I know there is a product that can assist you on your condition. you can try to search under Medela brand. you can gradually change yr FM. One feed per day, then slowly increase to two feed and so on. Monitor if he reject, otherwise you have succeed.