When you draw a face there are lots of tools to help. One I think of is using the width of the person’s eyes. It is not always right but it gives you an idea… All this is approximate…
One eye width between the eyes..
Two eye width for length of nose
Two eye widths for width of mouth
Three eye widths centre of eye to corner of mouth.
And one eye width from bottom of mouth to chin….
One eye width for bottom of nose.
Also if you draw a line down from the centre of the pupil it should hit the corner of the mouth.
Remember, this is just approximate. It depends on the face you are drawing, not measurements. Everyone is different! Also if you look closely faces are lopsided. One side of a face is often bigger than the other!