According to the Mirage Service Manual, the correct belt tension is determined by its resonant tone.
Just like a guitar string, the tone changes pitch according to tension. The dealer would use the MUT3 tool to measure it, and to get the tension right.
Now, since I do not own a MUT3, here is a different way of doing it:
Pluck the belt firmly where it runs under the engine mount, just as if it were a guitar or any other string instrument.
See the picture for details.
This produces a tone with a pitch proportional to its tension.
Play the audio file for comparison, and adjust the belt tension until its pitch matches that of the audio file. You may want to put the sound file on your mobile phone, so it can be played where and when you need it.
The sound file was recorded from a Mirage with round about 25.000km on it, being about 15.000 miles.
MP3 audio file: mirage-Belt-tension-plucking-sound.mp3