Yes, that is the traditional rule for the short sling: from the fingertip to the elbow. For my arm that is also 18.5". The measures of 20, 30 and 40" are an average approach to remember easily, but each slinger have to measure the slings in relation to the spread of his arms (to avoid erroneous measurments from chubby guys
) . For the medium one the distance would be from the fingertip to the shoulder, that in my case is 29,3", and for the long sling would be from the fingertip to the opposed shoulder, that in my case is 43". Well, I´m sure there are others slightly different rules
and it would be interesting to know them to extend our knowledge on traditions. If somebody knows them, please tell us.