Hi! It’s quite clear that you are required to contact Approach (if there’s any) when you are below FL180 and within 50nm (also for Tower below 10000ft and within 25nm).
However, there are some things that are a bit unclear to me. As STARs and APPs are rarely in a straight line, do these guideline distances for the 1) total distance you need to travel to destination OR 2) straight-line distance to airport?
Right now I use more often use altitude as the mark to call in, more so than the distance. Also if it’s a busy airspace, I’d most likely call in earlier. Thanks!