How to pick a braking marker for a first GTA VI route
A usable braking marker stays in place and is visible at speed. When you scout your first route, look for a sign, an obvious seam or a building edge before the turn. Pick a backup if traffic commonly hides the first one.
Keep the car and approach the same while testing. Move the marker earlier if you run wide repeatedly, then write down where you started braking and what changed about your exit. That record is more informative than a lap time by itself.