Bioplast labret studs with titanium stars

Labret studs go in from the back of the ear and are secured by a 3mm push-in star. The flat back makes these particularly comfortable to sleep in.

Length This sort of labret stud does not adjust so you need to choose the length of the post.
How do I work out the length?

1.2mm/16 gauge post
The same thickness as a standard piercing stud