I started a thread in the autograph forum about this:
http://net54baseball.com/showthread.php?t=152935
I had no idea what "TTM" meant until I opened this thread.
I think my success ratio was a bit higher than 30% -- more like 50-60% -- because I chose players based on the realistic probability they would respond to the request. There used to be a list I would reference and it was very accurate, down to the average turnaround time.
The downside was getting auto's from average players (i.e. Adam Kennedy, Mark Wohlers).
I did get some good ones: Tim Hudson, Mike Mussina, etc.