In giving potassium iodide I have, for many years, taught the doctrine that there is no such thing as a fixed or arbitrary dosage. When indicated, give it not with iodism as a guide, but until either the disease yields, or until very large doses—seven, eight, or nine hundred grains daily—without any modification of symptoms, satisfies you that its further employment is useless. To recur to iodism, that stumbling-block in the pathway of so many. Iodism should be a very rare obstacle to the successful treatment of nervous syphilitics. I have had to discontinue the drug in only two cases during the past six years, a success due, I believe, entirely to my method of giving the iodide, which is as follows: I always use the drug in the form of the saturated solution, for convenience and accuracy. I am careful to get a pure drug from a reputable chemist. Beginning with twenty drops (never less) after meals, the dose is increased three, five, or ten drops daily, according to the urgency or gravity of conditions. The vehicle is changed every third day, the first two days water, the next two milk, the next Vichy, then Apollinaris, then back and through the same cycle again and again, the amount of the vehicle being increased pari passu with the increase of the drug. Should iodism develop to an annoying or extreme degree, I stop for two days and then start just where I stopped, doubling the ratio of daily increase. Your patient will be thirty or forty grains daily further along before iodism recurs, employing this method. The addition of essence of pepsin in two or three-drachm doses after the iodide seems to be of decided service at times. The rennet mixture advocated by some writers is a vile mess, in my judgment, and is but rarely tolerated for more than a few days. Iodism is much more apt to occur from the continued use of a fixed dose, although a moderate one, than when giving it by the progressive method. I have seen iodism from continuous 20 gr. doses disappear at forty or fifty grains—paradoxical as this statement may appear. Following this method I have repeatedly given six hundred grains and more daily without any special inconvenience, and in one patient affected with meningomyelitis, a daily quantity of 1245 drops of the saturated solution was reached without untoward symptoms.
source: A DIAGNOSTIC SYNDROME FOR INTRACRANIAL SYPHILIS. WITH REMARKS UPON PROGNOSIS AND TREATMENT.. By WM. BROADDUS PRITCHARD., Medical Record, Volume 65, 1904
grain~ 64.79891 mg, 1⁄7000 of a pound (wikipedia)
more on the myth of iodism here: