This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Mixing The Developer. To mix the developer take of Solution No. 2, four ounces, and as each ounce of this solution contains five grains of pyro, the four ounces will contain twenty grains. Add to this, two ounces of Solution No. 3, and two ounces of Solution No. 4, making a total developing solution of eight ounces. You now have eight ounces of solution, and in this are twenty grains of pyro; therefore, the number of grains of pyro per ounce of solution will be one-eighth of twenty, or two and one-half grains per ounce. Therefore, the grain strength of this developer is two and one-half, and the factor for this developer is twelve.
Finding The Factor. We found the factor in the following manner : - A normally exposed plate was placed in this developing solution (the temperature being sixty-five degrees Fahr.), and the image appeared in exactly twenty seconds. The development was completed in exactly four minutes, or two hundred and forty seconds. Divide two hundred and forty (the total length of time), by twenty (the time required for the image to appear), and this gives a result of twelve. Thus twelve is the factor for this developer, and all plates developed in this bath will develop in approximately four minutes.
A Ten-Minute Developer. If you desire a ten-minute developer, take the above solution (eight ounces), and add to it an equal bulk of water (eight ounces). You will then have a total bulk of solution amounting to sixteen ounces, in which is distributed twenty grains of pyro. Dividing the twenty by sixteen gives one and one-fourth grains of pyro per ounce of solution. With the developer at sixty-five degrees Fahr., and a normally exposed negative, we found the image to appear in twenty-five seconds, and it was fully developed in six hundred seconds (ten minutes); therefore, six hundred divided by twenty-five gives twenty-four, making the factor for this particular solution twenty-four, and a plate developed in a solution diluted as above will develop in approximately ten minutes.
A Twenty-Minute Developer. A twenty-minute developer is secured by taking the above mentioned developing solution (eight ounces), and adding to the developer double the quantity of water (sixteen ounces), making twenty-four ounces of solution in which you have twenty grains of pyro. Divide this twenty by twenty-four, and the result will be five-sixths; therefore, you have five-sixths of a grain of pyro per ounce of solution. We found the image on a normally exposed plate to appear in this developer when the solution was at sixty-five degrees Fahr., in thirty seconds, and the development was completed in 1170 seconds (nineteen and one-half minutes). The factor for this developer is, therefore, 1/30 of 1170, or thirty-nine. Although the exact time of development of this developer is nineteen and one-half minutes, the latitude for a developer of this dilution is so great that no harm will be done in developing the plate for twenty minutes; and for tank development, where you do not desire to look at the plate after it is placed in the developer, you will find that leaving it in this solution from twenty to twenty-two minutes will give you good development for negatives which might vary in exposure, some being a little under-timed, some normally exposed, and others over-exposed.