It is common for a surgeon or a pediatrician to say to an anesthesiologist (sometimes in a crowded room), "We're bringing a four-year-old child up right away for emergency surgery. What do you want him to have?" It is too difficult to commit yearly dosage levels to memory, and an easily learned formula is a distinct help to the anesthesiologist and to the patient as well. Formulas need no defense in medicine or elsewhere, so long as their user remembers to consider other factors occasionally, such as (in this instance) obesity, malnutrition, head injuries, somnolence, and so on. In the majority of cases the formula (or table, if the anesthesiologist has one at hand) may be used without deviation. It must be pointed out that older rules for obtaining children's doses from adult values do not apply well to drugs used for preanesthetic medication.
It is helpful, too, to know
COLE F. Pediatric Formulas for the Anesthesiologist. AMA Am J Dis Child. 1957;94(6):672-673. doi:10.1001/archpedi.1957.04030070084009