As simplified as this may sound - the kitties will let you know.
One day you will go to feed them their second feeding and you will notice there is leftover food. That will be a sure sign you can cut back a bit. Maybe do smaller portions for their feedings or cut back on the amount of feedings in a day.
You are right as to the amount for adult cats. I never really stress over it. I feed my guys what they want to eat. Most times it amounts to about a small 5.5 ounce can each a day. But I have some big eaters (translate that to big cats) who probably eat closer to twice that.
I have found with all the kittens I've raised that around the 6 - 9 month stage they start to slow down with the amount of food they eat. Kittens need a larger amount of food just because of their pure rambunctiousness. They get the zoomies so many times a day they burn off a ton of calories. Once they start maturing into adulthood the zoomies slow a bit too. Dart, at almost six months, is starting to not eat as much as he was before. He can still pack away more food in a day than three of my adult kitties at three years old. With my seven month old raw eaters I feed approx 2 cups of raw twice a day plus a large can of Wellness in the morning. Occasionally I add another can of Wellness at night.