Feeding by the flake isn't very accurate. Some bales have flakes that weigh 1 pound, while others weigh near 5. I would recommend getting a hay scale and attaching a bucket to it to weigh your mini's hay. It is real easy to overfeed these little guys!
I work at a barn that has 2 mini's, and they split 4 pounds of hay per day, divided into two feedings. Each mini eats only 1 pound per feeding, adding up to a grand total of 2 pounds of hay per horse per day. They don't get grain, but the hay is very high quality and they are hardy little horses.
I petsit my neighbors goats, and they are the pickiest eaters! They don't even eat the hay that falls out of their manger onto the floor. Horses are less wasteful than that!