I just happened to perform this adjustment recently, and here's how I did it...
First, I bought a test tape. There are plenty of places that sell them; I got mine from
Studio Sound Electronics. These tapes contain several test tones, including one that's meant for speed adjustment. Mine uses 1kHz.
The next step involved using my computer to generate a 1kHz tone. Most any audio editing software can do this; I use one called DC6. I generated the tone, and started it playing.
Now, the connections. I connected one channel from my computer's sound card output to one channel of my Pimeta (on topic at last!). I plugged one channel of the tape deck's output into the other channel of the Pimeta. And, of course, I plugged my headphones into the Pimeta's output, and turned everything on.
Now, all I had to do was play the test tape, and adjust the deck's speed until the two tones matched. It took a little fiddling with the sound card and tape deck level controls, but it worked quite nicely.
The accuracy of the adjustment should be quite good, assuming the computer's tone is highly accurate, which I believe it is. Listening tests seem to confirm this so far.