Former hour- record holder Graeme Obree designed a track bike with a really narrow Q, to make himself more aerodynamic.
His feet were so close together, there was no room for chainstays, so his raised them out of the way.
Generally, modern systems(like MTB triples) have a really wide Q compared to older/vintage models.
If you want to chose a particular width, select it on the basis of hip-width, leg and crank length, and aerodynamic requirements. There are no guidelines, and most people take what thye are given.