Even if the tester is properly calibrated, it is hard to estimate remaining life of any tube that tests strong--at or near the expected value. That is because of the way tubes perform with age. When brand new, a tube will typically test very strong--well in excess of expected value, but performance will quickly fall to near or slightly above the expected value. The vast majority of the tube's life will be at or near this plateau. and only near the end of its life will its performance begin to decline. The fall off from there can be quite rapid. When you get a tube that tests strong, you really don't know where along the broad plateau that the tube is at--is it at the beginning or is it close to where performance will drop dramatically? It is hard to tell. If the tube is testing weak or marginal, it probably is well into the end phase--that is about all you can tell from test results about remaining life.
If a tube is tested on a modern machine like an Amplitrex, the tube is being tested under stress conditions that are better at revealing the age of the tube. I've seen quite a few tubes that tested very strong on machines like a TV-7 that did not look quite so good on an Amplitrex. That tester is sort of the gold standard and most tube sellers who claim that they test their tubes will proudly claim use of that tester if they use it (some post screen shots of the readout). Amplitrex does not use a number scale, like xx/xx, so results given with such number mean that some other tester is being used.
