What is the rule for predicting height of a child based on parents?

None. Typically a child's adult height will be somewhere between those of his parents.
Average +/- The common rule we start with averages the height of the parents and adds 2.5 inches if a boy and subtracts 2.5 inches if a girl. This gives a rough figure with many possible exceptions. The best predictor becomes the pattern of growth the kid exhibits during childhood and the age s/he starts into puberty. (growth charts on their docs records)