Most cartridge manufacturers have a recommended tracking force range, which allows for a bit of leeway plus/minus. I have used a Shure SFG-2 gauge for more than three decades, and have found it to be very accurate when checked against much more expensive digital gauges.
I am confused a bit by your comment about making final tracking force adjustments by ear. Are you referring to VTA (vertical tracking angle)? One DOES make final VTA adjustments by ear (after initially setting the tonearm to level), but I have never heard of making tracking force adjustments by ear. Seems to me that would mean there is no need for accuracy from the tracking force gauge.
I am confused a bit by your comment about making final tracking force adjustments by ear. Are you referring to VTA (vertical tracking angle)? One DOES make final VTA adjustments by ear (after initially setting the tonearm to level), but I have never heard of making tracking force adjustments by ear. Seems to me that would mean there is no need for accuracy from the tracking force gauge.