This is the one I have - never used it yet - kept it for reference.
Mini Growth Chart
Projected Height 28" 29" 30" 31" 32" 33" 34" 36"
At Maturity
Birth 18.2 18.9 19.5 20.2 20.8 21.5 22.0 23.4
6 months 23.8 24.7 25.5 26.4 27.2 28.0 29.0 30.5
12 months 25.2 26.1 27.0 28.0 28.8 29.7 30.5 32.5
18 months 26.4 27.3 28.1 29.2 30.0 31.0 32.0 33.9
24 months 27.2 28.0 29.0 30.0 31.0 32.0 33.0 35.0
To use the above chart to determine mature height for a foal, measure the height of the miniature foal at a given interval such as birth, 6, 12, 18 or 24 months of age. Located the age interval on the left hand column. Then scroll across the age interval until one finds the height of the miniature foal. Once the height is located scroll up to the top line for mature height of the miniature horse.
Example: If a mini foal is 19.5 inches at birth the mature height of the miniature horse should be 30 inches.
The above chart is intended as a reference only and will have slight variations based on the genetics, nutrition, care, and health of the individual miniature horse foal.