Ammeter voltage drop

The thing is you CAN NOT accurately measure low values of resistance with a multimeter. This varies with the quality of the meter, but below two or one ohms, you are getting flakey, and something like Horrid Freight/ other Chinese import, maybe higher than that

The only real thing you need to be concerned about is total voltage drop when charging hard, AKA with 20, 30, 40 A going through the circuit, THE ENTIRE CIRCUIT, which includes the fuse link----the connection at the start relay "big stud"---the wire end terminals----the bulkhead connector terminals---the wire terminals at the ammeter---of course the ammeter itself, and so on

This figure varies with opinion, but "Let's say" you have a 50A alternator operating at that output, you have a load on the battery to simulate heavy load, if you have anything less than 2V between alternator output stud and the battery POS stud, I'd say "call er good."

The bit issue with the ammeter is that the studs are not soldered/ brazed/ welded to the shunt/ guts. These connections can get hot with extreme current especially with something "got loose" and a little corrosion, etc. If you want to improve this, take it apart, clean it up and using proper flux, solder the studs to the shunt with silver based solder