The zero value should not change when using the transfer from 2x PX625 1,35V (Mercury) to the V206A adapter 2x SR44 1,55V (Silveroxide) batteries.
The build in Schottky diode brings it back to a stable 2,7V operating voltage. Comparing the EV F/t iso values over a wide range they are within 1/3F stop of my Leica M7 metering system which is also about the same for my M645 pro AE prisma metering and even on the C.V. Bessa III 667, non TTL exposure meter. Values from 4s - 1/1000s (1/4000s (EOS) ) I can check. The very long exposure possibility from a Gossen Lunasix-3 / (3S) -4EV +20EV is over every camera metering system I have. Concerning the reciproke correction of most films it is not a big deal because +/- 20% in this area you do not see much difference.