https://forums.pelicanparts.com/porsche-924-944-968-technical-forum/421347-balance-belt-idler-misinformation.html
I found this post in my searches, which has factory illustrations indicating 0.5mm is between the idle roller and the belt below it, and it every so lightly touching the belt above it.
Problem is no matter the own I place the roller that spec is out by 2mm.