I find it a little bit odd that people who have passed exams on aircraft performance and other pretty technical things to do with aviation get hung up on descent management. All you need is a rough idea of how your aircraft behaves when descending and/or slowing down, adjust it for speed and wind component, then keep an eye on how it’s going and tweak as necessary. At economy speeds in still air, mine will do ~3.3 miles per 1,000’ and loses ~20kts/nm in level flight at idle. Work that back from where you want to end up and there you go...