I received this suggestion chart from another Miniature Horse breeder.
It may at least give you a guideline to start with.

Age in Days
1
2
3-5
6-8
9-11
12-14
15 & up
Feeding per Day
16
14
12
10
8
7
5
Time Between
Feedings
1.5
1.5
2
2
3
3
4