If you want to do these in the Sports Photo Automation plugin then there is a pretty good way. However, that involves shooting full body with feet and allowing it to crop 3/4. It can use subject detection to recognize both the face and feet. Then you can have a PSD SPA template set up to place the face in a certain location and scale to put the feet off canvas an exact amount below the bottom. This is the most accurate way to crop 3/4.
With the Face Crop plugin, there is no way to tell it where to place the feet. You can create a preset to scale down so the head is smaller and create 3/4, or even full length. The smaller you size the head, the more of the persons body that will fit on the canvas. However, the exact cutoff point will vary more between images. For example, you may have 2 people with the exact same size of heads but on person is 6′ 4″ and the other is 5′ 2″. If the heads are places and scaled in both of those images the same, the taller person’s feet will be much lower than the shorter persons feet. So that will make the crop point different on the bottom.
In face Crop, click on the home button. Then click on the Create Presets button. You can adjust the face size and top margin. Then click on test preset to see what it does. When you get something you like you can then save the preset. however, as mentioned, the bottom cutoff point may vary image to image. The closer to full length, the more noticeable this variation becomes.
So first try this out in face Crop. If the bottom cutoff variation is too much then you’ll probably want to do the route of setting up a template for the Sports Photo Automation plugin instead. For that, you really need to shoot with the feet in the image though. Then the plugin can crop accurately 3/4 by placing the feet off canvas to a specific location.
Let me know after you have tested and if you want to go the route of using SPA. Then we can discuss more details as that is a little more involved with the setup.