Formula is weird. I get around 1.1-1.3 with changing the values (not the age and rank).

So this means no matter how much I am good and how much I study per day I will improve at about a stone per year?

However I tried it for some young Chinese 10 year old kid and I got up to 2.75 stones per year, meaning around the age of 15 they would turn pro which does make sense.