Every percentage in Slope is measured, not guessed. Here is exactly what happens between you opening the caddie and it saying "41% green".
Each club has a picture: how far it carries for you, how much that varies, and how far left or right it tends to miss. Until you have logged five shots with a club, the picture is a typical one for a golfer with your carries and scoring average. After that, it is yours, and it keeps sharpening. A few taps after the caddie's call ("green", "in play", "sand", "trees", "water") tell it whether its picture of a club is too tight or too loose.
The green, the water, the sand and the line of play come from the course map. Anything off the line by more than about 30 yards counts as trees. When you show the caddie your lie, it adds what GPS can't see: the lie itself, the branches, the slope, the gap, and it proposes the shots a caddie would (a punch under the pine, a high cut over the corner, a pitch out). It never gives you a number.
For each club and aim that makes sense, Slope plays the shot a few hundred times with your picture of that club, including the occasional fat or thin that every amateur has. It counts where the balls finish and prices each finish by how many strokes a golfer like you needs from there. The option with the fewest strokes to hole out is the caddie's pick; the others are shown so you can choose your own risk.
Your Twin plays the same model around a whole course and compares what the caddie quoted with where the balls actually went. If those drift apart, the app tells you. It also tunes itself to your real scoring average before it tests any change, so "driver plus ten yards is worth 0.8 strokes" is measured against a twin that shoots what you shoot.
Roll, gusts, firmness and bad bounces. Treat the odds as a strong prior, and the lie reads as a caddie's eye, not a promise. The engine runs entirely on your phone; only the lie reads and the written advice use a server.