There is no correct compensation, as the error will vary upon your light intensity. The brighter the light, the greater the meter error.
However, all is not lost. U have a number of options:
1) Purchase a WEINCELL W990120, which is the correct voltage and size replacement for the PX625.
2) Get an (expensive) adapter called an MR-9, which will allow U to use a silver oxide 386 battery in place of a PX625 mercury battery. Link: http://www.criscam.com/mercury_battery_adapters.php I have one of these and it works well.
3) Make your own adapter cheaply, which uses a zinc-air #675 hearing aid battery. Link on how to do it for about $1: http://rick_oleson.tripod.com/index-111.html Myself, I've used some tin foil and a coin, in a pinch.
Now, if U have ISO 400 film, and U have a camera stuck on ISO 100, then U need to underexpose by 2-stops. Negative (print) film tolerates overexposure well, so any slight underexposure from what the meter says should give U good results. Positive (slide) film does not tolerate deviations from proper exposure very well and only slight underexposure. If U do not have a reliable meter and cannot use a handheld meter, I would not recommend using slide film.
Example of underexposure: If U have a meter that says U should shoot at 1/125 at f/5.6, to underexpose by two stops, U either need a faster shutter speed (so from 1/125 to 1/500) or a smaller aperture (f/5.6 to f/11). Or U could do both (1/250 at f/8).