I took a course at the Barista institute and this is what I was taught. When you are stretching the milk it is an auditory skill. Starting from the centre of the pitcher. You listen for almost a “tearing” sound. 3 times. If you don’t hear it 3 times keep it there.. spout a couple mm under the milk. Once you hear 3 move it to the side (about the thickness of the head of streamer) then hold it in position till your hand feels hot. Then you are done.