The correct belt tension is determined by its resonant tone.
Just like a guitar string, the tone has a pitch proportional to tension.
Here is a quick and easy way of getting it 100% correct:
Pluck the belt firmly as in the picture below where it runs under the engine mount, just as if it were a guitar string. This produces a tone.
Play the audio file (below) for comparison, and adjust the belt tension until its pitch matches.
You may want to put the sound file on your mobile phone, for playing it when and where you need it.
MP3 audio file: mirage-Belt-tension-plucking-sound.mp3
Happy twanging
Foama
The correct belt for Mirage with A/C has 6 ribs and a length of 1240mm.
Some makers offer a length of 1238mm which fits just as good. Any shorter and it becomes difficult to install.
Part number 6PK1238 or 6PK1240 (non metric: 488K6)
Our cars were made adhering to universal (metric) standards as harmonized by the International Standards Organization (ISO). In the last three remaining non-metric countries, namely Liberia, North Korea, and the USA, the 6PK1240 may be sold as 488K6. See bottom picture, where the belt has both printed on it.
Picture of the OEM belt 6PK1240 (Type 488K6 in some non-metric countries)