What you want is a meter that may be off, but is consistent. You can use that with confidence. Doesn't matter what it says-- if you know the "index error", just apply that each time. What really matters is to observe any change from last time.
I have no problem with my cheapo hydrometer eg. It does not need to be accurate and temp compensated. It just has to tell me if anything is out of whack since the last reading.
Same thing with the multimeter. Only thing there is the battery getting low makes the voltage reading wrong. It happens slowly, so you can be fooled for a while until you catch on that it is the battery.