That Bruce Jones video is gold. So are the numerous tips from the guys here.
It makes sense that if your rack supports are level and your blank is equal thickness on both sides that you can go from there but it isn’t that easy as you often introduce your own asymmetries as you shape.
Those siting sticks give a perfect reference. I’ve seen horizontal lines drawn on the back wall of a shaping room to use as a reference.
There are also adjustable rocker sticks that help you keep track if you place it on the stringer to get your curve and then check at intervals on either side of the stringer - like 3" to each side, 6", 9", etc.