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.
Actual Illuminating Space. First measure approximately the size of the window space used, or if more than one window is used combine the measurements of all. If a portion of the window is cut off with blinds, you must only consider the actual space supplying illumination, multiplying the width by the length of the window. For example: If you have two windows, each 3x5 feet, one of these windows would measure 15 feet; thus the two supply 30 square feet. This constitutes your illuminating space.
Space To Be Illuminated. If your room measures 12 by 18 feet, multiplying the width by the length gives you 216 feet. This constitutes the floor space. If more than one room is admitted you must, of course, consider the size of the combined floor space, just the same as you did the combined window space. Divide the combined floor space (216 ft.) by the combined window space, (30 ft.) and the result of this (which is 7) you multiply by eight times the amount of exposure necessary for outdoor work in the shade (for all practical purposes you can calculate on 1/2 second for bright weather and longer for dull weather), and the result (28 sec.) is the amount of your exposure necessary for the interior.
Stops. The stops will range in the same order for interiors as exteriors. If with a No. 16 stop under the above conditions, 28 seconds exposure is sufficient for medium light walls, the next stop, U. S. 32, would require 2 times this exposure, or 56 seconds. For a No. 64 stop it would be correct to give 2 times the exposure required with a 32 stop, or 112 seconds, or approximately two minutes.
Requirements. The principal requirements are, therefore, that you judge accurately the necessary exposure for outdoors with the same stop as you are using indoors. This supplies the factor by which to obtain measurements of the light for indoors. By this mathematical method of figuring you can judge fairly accurately the required exposure, but you should not rely entirely upon this rule; in fact, you should train the eye to measure the light by the appearance of the image upon the ground-glass. Combining the two, however, you have a good guide to follow, as the mathematical method will give you an approximate estimate, while your final judgment should be based upon the appearance of the image upon the ground-glass. These combined will serve well until you have had sufficient experience to judge the exposure by sight.