How accurate is the height approximation of your children when basing it off of his/her parent's height?

well i am taller than my parents my mother is 5'4 and my father is 5'8 and i am 6'3". The formula the dr. uses is just a guess based on averages. They double the child’s height at two years old and add three inches if it is a boy . But like you said that does not take into account grand parents height nutrition or even excersie or lack there of which can all affect height. In my case my great grand father was over 6 feet and so are some of my uncles , and on the other side of the family my grand father was 6 foot so that is where I get my height from. My brother is 21 and is only 5’8 so he won’t be 6’ so it all depends.