How to interpret the readings from tube tester?

Generally 104+ on a TV-7 indicates new spec for a 6SN7. (2600 umhos). 98/99 - a little lower than "new" but still very good, may be new in fact, since not all tubes (nor testers) will always test exactly the same.

BTW, forget Upscale Audio for 6SN7s (and 6DJ8/6922s) unless you have bought gear from them. They will only sell ho-hum NOS tubes (e.g. Russian crap) in those types, unless you have bought gear from them that uses those tubes, then you get the "special cookie jar" tubes. No other sellers (including myself) have this sort of first class/second class when it comes to buying and selling.

All IMHO of course,
-Ed

RE: Calibration: Well, one knows it's calibrated from calibrating it (or having it done) from time to time. one of mine is easy to calibrate (Hickok 580), the other (TV-7) is more complex. But both are calibrated.

RE: Matching - agreed, the only way to really match is either with the tubes in the circuit in question (tricky for most users) or on a curve tracer, like the Tek 570. The 3 elements of matching (or at least 3 most common/crucial elements): Plate resistance (impedance), plate current, transconductance. Most testers only test one of the three. Some test 2. (current and transconductance). There's far more to it than can be written here, but suffice to say most of it is not really relavent in terms of getting good sound from tube gear. Microphonics (which are difficult to measure and only can be done in an audio circuit) are far more important in terms of what you hear.
-Ed