What is the correct tip to lip depth of tube insertion for correct placement mid trachea?

The approximate depth of insertion of the endotracheal tube from the middle of the upper

lip, in centimetres, can be calculated as weight in kg + 6 cm. (Peterson J, Johnson N, Deakins K, Wilson-Costello D, Jelovsek JE, Chatburn R.

Accuracy of the 7-8-9 Rule for endotracheal tube placement in the neonate. J

Perinatol 2006;26:333-6.)