1. remember that boobs are not perfect spheres( kind of like tear drop shaped on a flat chest)
2. the bottom of the boobs should always line up with the shoulders( correct perspective) so parallel lines should be created.
3. the tips(nipples) of the boobs typically end up around half way done the bicep( so you can calculate the distance based on that.)
4. there will be a crease of the boob that will connect with the armpit area.
* these are only a few of the things to consider*
* I wish someone would have told me them before I had to figure it out myself*