Well, personally, I see nothing on the block diagram above the instructions that suggests or infer that A represents the VTVM output reading at line-out. The input signal strength was very specific, why not the output too. Furthermore, the input was set to db. But the output is supposed to be measured with a VTVM which reads volts. Since signal generator outputs are usually either uV or dbm (watts), then are we supposed to convert from dbm (watts) to the microvolts representation of -60db? Why not put the desired exact microvolt reading into the "adjust-for" field? Finally, something to consider is that the block shows the input signal to inject into the (mic) input, not line-input as you stated. This is a very important distinction because microphones typically have very low signal strengths, which is why the block diagram shows the signal passing through a mic amp and then a flat amp. After being amplified by 2 amplifiers, it's fair to say that the output signal will be far more robust than the input signal. Let me also include the following suggestion which I think may make more sense to you... if the microphone input signal of -60db was to be kept at the same level at the line-output (provided you can even measure -60db), then any device connected to the line-out jacks would not even register such a low signal, which should actually be (reference) -10dbv (.316v) consumer, 0dbu (.775v) professional, or (line) -10db consumer or +4dbu (1.23v) professional. In other words, the mic input level needs to be "boosted" by 60db in order to have the proper line-out signal strength. So if we presume that we are going to use the consumer 0dbv reference or line level signal that we are going to shoot for. In that case, we need to see .316v at line-output.
But lets move on from that discussion and look at a preceeding measurement. The VU meter adjustment. As you can see, they specify that when playing a MTT-112B test tape, the VU meter should read (+4db). I doubt that you have an MTT-112b test tape so you can instead inject a -20db signal into line-in in which case, the expected VU meter should read 0db. Since both settings prescribes adjusting the same pots, one can surmise that either adjustment is fine for setting VU meter. Now moving on, we also see that in the recording level adjustment, the same test tape is prescribed for measuring the output and the expected reading should be (3 or 4db lower than "A", depending on which SM you wish to follow). Lets say 3db. Now, I hope I haven't lost you but if in the VU meter adjustment, we are expecting +4db using that tape, and if in the record level adjustment we expect 3db less than (A), then doesn't it stand to reason that A = +4db + 3db? In the VU meter adjustment, the VU meter itself is what is being calibrated. However in the Rec level adjustment, you are measuring at line-out. If you trust the VU meter, you can use that to set at +7db with -60db input at mic. However, if you would prefer to set using a DMM (presuming nobody has VTVM anymore), then you should first probably find out what the line-out level is at with meters at +4db and -20db at line-in, and adjust your expected output from that reading by +3db over.
As to your last question regarding the VU meter adjustment, it appears to me that:
- You set VU meter by injecting -20db signal at line-in, and adjust to read +4db on VU meter
- You set VU meter by playing the MTT-112b test tape, and adjust to read +4db on VU meter
Choose 1 or the other.