I don't what understand what you are trying to prove by measuring the negative terminal for voltage. If you are verifying the ground is good you should be measuring resistance in ohms.
Tom, it is a voltage drop test. Doing voltage ground to ground will show a loss of voltage through a cable under load. It is the correct way to test it. If you take the volt meter and place one lead end on the battery terminal and the other at the end of the cable you will see the amount of voltage loss through the cable. Anything over .5 volts needs to be looked at.
There is a link I posted in my following post on this procedure. The issue with a resistance test is the circuit must be isolated to get accurate readings.
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