Hi Luis – Only my opinion, but I don’t think you should try to manipulate the lips to get all the red inside the mouthpiece if they don’t fit. IMO what happens in the centre of the lips between your corners is more important than what’s going on around that. When you do the bluZZ with the lips, make a note of where the lips meet each other and try and place the mouthpiece approximately central on that – so that the gap between top and bottom lips is sits approximately in the centre of the mouthpiece. When you do ahh, ooh – make sure that the lips are neutral when you place the mouthpiece and not overly puckered. You want free, unhindered vibration of the lip tissue and a full sound – I’d go for wherever your low C sounds the most vibrant and full and work with that.