Hi Hugo,
I agree the Ah-ooh without bringing any tension to the aperture corners is quite wide. As Greg says it’s about the size used for pedal tones. Also Greg says the only good tension is at the aperture corners. Visualize your aperture as a diamond shape, slightly flattened and bring tension to the aperture corners. This will bring the corners in from the sides and push the center of the lips slightly forward. This is all done inside the cup of the MP. You should strive to feel it happening. I was told along time ago from someone I don’t remember that if you play the leadpipe alone with the correct set, that you should be close to sounding a written first space F on the B-flat trpt. It has worked for me for years.

Good luck,
Brian