Endotracheal tube level is –
"Ideally, the tube tip should be 5 ±2 cm from the carina when the head and neck are in neutral position".
___ Fundamental of anaesthesia

Neck flexion causes a 2 cm descent of endotracheal tube towards carina, and neck extension causes a 2cm ascent of trachial tube tip from the carina. Therefore correct position of tip of the tube is :-

5 ±2 cm from carina → When neck is neutral
3 ±2 cm from carina → When neck is flexed
7 ±2 cm from carina → When neck is extended
5 cms above xiphisternum
4 cms above carnia
1 cm above carnia
At carnia