The bar obscure hub method or the elbow to finger tip method both will get you into the ballpark. Neither method takes into account the vast differences in handlebar reach and drop. Brifters also add another layer of complexity as they tend to extend farther than old school brake levers. That can account for 3 to 4cm variable even before thinking about stem length. Miles on the bike is the best method for finding what is correct extension that fits you.