A more pertinent question is how does one know the tube tester is accurate/calibrated?
As far as what is left in the tube, the Gm readings will give you a feel of the tube, not any with definite certainty of what's left in it. A good way to figure out how much life is left, the tube should be tested at a lower voltage. For example, test the 6SN7 at both 6 volts and 5 volts. If the Gm remains constant during the switch, the tube still has a good deal of life left. Some tube testers have a switch that reduces the voltage between 5% and 10% during a test - if the reading remains constant, it indicates a good tube; this way you don't have to worry about interpreting specific mho numbers - just the change (or lack of).
Again, what comes out of the amp can be totally independent of what is tested, and you cannot say with absolute confidence that a higher value tube will sound better or last longer than a lower one. It's a crapshoot, buying old tubes.
As far as what is left in the tube, the Gm readings will give you a feel of the tube, not any with definite certainty of what's left in it. A good way to figure out how much life is left, the tube should be tested at a lower voltage. For example, test the 6SN7 at both 6 volts and 5 volts. If the Gm remains constant during the switch, the tube still has a good deal of life left. Some tube testers have a switch that reduces the voltage between 5% and 10% during a test - if the reading remains constant, it indicates a good tube; this way you don't have to worry about interpreting specific mho numbers - just the change (or lack of).
Again, what comes out of the amp can be totally independent of what is tested, and you cannot say with absolute confidence that a higher value tube will sound better or last longer than a lower one. It's a crapshoot, buying old tubes.