The exposure correction needed for a filter depends on the light, the subject, and the film. A certain amount of bracketing and experimenting is expected. Most filter manufacturers provide a list of filter factors for their products which is at least a starting point for the exposure correction. (Divide the unfiltered exposure index for your film by the filter factor to get the effective exposure index to use with the filter.) For instance, Kodak recommends a filter factor of 2 in daylight and 1.5 in tungsten light with panchromatic film for their No. 8 medium yellow filter. But if you were using a film with high red sensitivity, you would need to reduce these factors, and with orthochromatic film you would have to increase them. Filters vary from one manufacturer to another, but Kodak recommends a filter factor of 2 (one stop) in daylight for their No. 12 (Minus Yellow) filter. They do not list filter factors for their No. 21, but looking at the absorption curves and comparing it to No. 23A, I would estimate a filter factor of 4 (two stops) or a shade more for that filter.
Note that a No. 12 filter produces much darker skies than the usual No. 8 filter. It also gives greater haze penetration. Filters are generally used to produce additional contrast, lightening colors similar to the filter color and darkening the complementary color. Obviously, the subject color will affect the exposure. Here is where exposure can become tricky and bracketing can be wise. If you want to darken blues, you want to reduce their exposure in relation to the other colors. That is just what a yellow filter does. Other colors may not behave as expected, and different films can behave differently. The filter factors generally work fine to give contrast effects, but it can get tricky.
