Below are a few guidelines for portrait proportion, while drawing a typical adult head. There are a few standard view in portraiture, however, here in this post, you will be reading basics references to a person who is looking straight ahead. These generalities apply not just for your graphite drawings, but also as a rule of thumb for basic fast sketch in starting portrait oil paintings.
The eyes are about halfway from the chin to top. This is the usual starting point in portrait drawings. So always check back with reference from the chin. Make sure you don’t begin drawing the eyes level to high, and leaving forehead too small.
The inside corner of the eye lines up with the outer edge of the nostril on a vertical axis. There are times, when this point is very slightly overlap for people who have broader nose.
The corner of the mouth lines up with the inner corner of the iris on a vertical axis. Since the eyes are the most important features, its worth the time spent on placing the iris on the right position. Otherwise, the result of your subject may appear to have squint eyes, instead. This is true to most of the portrait sketches.
The lower part of the face from chin to base of nose measure about the same as the area from the base of nose to a point between the eyebrows above the bridge of the nose. Proportion will differ, if your subject is drawn with a laughing mouth. In reality, your subject would not be able to pose with a laughing mouth for hours, however, do you know that some artists practice drawing directly from a candid photograph in realistic portrait? So always keep at least 2 logical references, to maintain every features are in the correct alignment.
The eyes are usually separated by a further eye’s-width across the bridge of the nose. The only difficulty in this proportioning, is when your subject is wearing spectacles. The outline of frame, particularly small over shape, may tend to distracts your attention in justifying the gap correctly.
Ears are generally on a level with the nose. No artist wants their portrait to looks like elf, with ears drawn too high.
There is a simple illustration below that I have sketch out, based on the above-mentioned points. Image may not look perfect to be realistic, however, you may want to give feedback if it does not relate well to the context here.