A 3x factor does not add up mathematically to a 2 stop compensation-----a 2 stop compensation is a 4x factor, adding twice the amount of light with one stop, then adding twice the amount of light again with two stops. I don't know what the best factor actually would be for the 25A, I'm only commenting on the factor as it is being advertised in f/stops. It has to correlate to the fact that opening up one stop adds twice the amount of light as the previous stop----just something to keep in mind.

2^{1}= 2x factor

2^{2 }= 4x factor

2^{3 }= 8x factor

A 3x factor is an intermediate factor that adds 1 2/3 stops.

It's the 1:2 ratio that exists between each f/stop and each shutter speed on the lens. Opening up one stop adds twice the light, stopping down one stop cuts the light in half, therefore, if you open up 2 stops with the 25A filter, you are adding a 4x factor to the film.