The proper mercury batteries provide a consistent voltage to the end of their life, but the modern replacements gradually loose voltage. It generally starts off too high then reduces over time and the meter readings are correct for only a short time. Check out what appropriate replacements are available and work from there.
...unless the camera has a bridge circuit to equalize a different voltage, which this one has (I conclude that from lorenzo1910's reply). I have to admit that I don't know exactly how it works, but my similar LTL3 had very accurate readings for more than a year with one LR44. Maybe it'll become inaccurate after some more time, but with a battery that costs a few cents, that shouldn't be a problem. The only real replacement would be a WeinCell, but since it's zinc-air, it won't last nearly as long and is much more expensive.