The one we have used for decades on many horses and has not failed yet is, from the middle of the ankle to the elbow, turn the string around and from the middle of the elbow up, that is what most horses should grow to, within 1/2", unless injured or starved badly.
The most memorable was a coming 4 year old measured in February as 14.3 hands and due to grow to 15.2 hands.
Neither us or the vet thought that possible, he looked mature and square.
Measured again at the vets in June, measured then a full 15.2 hands and looked kind of lanky.
He later filled out.
I've done middle of the knee to coronet band. Whatever that is in inches is what they should mature to in hands. For both of my yearlings it's been within a 1/2". One went over and the other - well, he's still growing, but he's almost 6 so I'm thinking about done.
Measure from the ergot to elbow. Hold that length of string then swing the tape up from the elbow to the withers. The distance between the withers and the length of string above it is what the horse will grow. This is accurate on yearlings and older.