When you set the camera speed manually, shoot Tri X or HP5 at EI 50, or actually, I would recommend to shoot somewhere between EI 25 and 32 ( for me, real speed of these films is EI 250), for shooting with cameras with auto exposure, a rule of thumb is to point the camera at a typical scene, set the ISO to the desired value ( e.g. 250) and take the light reading. Then apply the filter and repeat the reading again. You are likely to note, that the internal light meter will only differ by 1,5-2 stops. Then, you have to "downgrade" the ISO in camera by the difference.
Example: you set Tri X to ISO 250 on your Hexar, you take the reading. The reading is 1/250 f 8.0. Then you put the filter on and repeat the reading - it should say something like: 1/250 f 4.5. Then you need to downgrade the ISO in camera from 250 to about 80.
Do not follow Colton's advice, as it will lead to underexposure.