As you know, a filter stops some of the exposing light while passing some. The Wratten #25 is the red tricolor used with the Wratten # 58 Green and the Wratten # 47B for color separation work and for haze penetration in aerial black & white photography. It was used to simulate night in black & white movies. It adds contrast and brings out clouds when used with black & white films. OK, its published filter factor is 8.

We handle a filter factor by multiplying the shutter speed without filter by the filter factor, in this case 8. Likewise, since each f-stop = a 2x change, a filter factor of 8 = 2x2x2 =8 meaning 3 f-stops compensation. We can also divide the ISO by 8, and reset the ISO to this calculated value and take an exposure reading at this reduced ISO.

Your camera sports a thru-the-lens metering system. The Wratten #25 will takes its toll, so the measurement will be about 3 f-stops less. This should be an OK reading and likely you will not need to take any additional action. In other words, the exposure system will compensate for the mounted Wratten #25.

Now comes the “but if”: A filter factor is a compensation derived based on a specific light source, usually sunlight. The filter factor will be slightly different for other light sources like tungsten. As a general rule, the differences are negligible. After all, the best we can likely set our camera is + / - 1/3 f-stop. I also want to tell you that the filter factor is also dependent on a particular film emulsion. This is because different emulsions have different spectral responses.

Now comes the rest of the story: Light meters also differ in their spectral response. Therefore, they will give different readings when tasked to measure daylight or tungsten etc. They surely will give different readings when tasked to measure the exposure through strong filters. The good news is, likely your readings will be in the ballpark -- meaning no special action required. However, be forewarned, the Pentax K-1000 might be fooled by the Wratten #25. Your approach should be to bracket your exposures. Making a test exposure series is also a wise approach.