Eg faces range from 12 to 18 ft away.
So if your DOF calcularion shows it will be between 12 and 18 feet, pick a face at about 14 feet away. This ensure all faces between the desired in focus range will be razor sharp.
Some posters said pick the mid point. In this case if you picked a face at 15 ft then those face at 12 ft might be a little less than razor sharp! As the new DOF might be about 13 to 19 ft - course it gets more complicated as distance has an effect on DOF.
Note as a relatively inexperienced shooter I may be wrong!
Always very happy to be corrected.
DOF doesn't work this way, it doesn't define a zone where everything is razor sharp, just "acceptably sharp". It will only be truly razor sharp at the distance you focus on, then it starts getting softer and softer the further you move away from that plane of focus. The point they become so soft they are no longer "acceptably sharp" is the edge of the DOF zone. In your example, the faces at 12 feet and 18 feet will be far from razor sharp, as they are on the edge of the DOF they will be barely acceptably sharp, whereas the face at 14 feet (which was focused on) will be as sharp as the lens can deliver.
Also, consider that DOF has several parameters, two of which are image size and viewing distance. DOF tables assume a 10x8 print at arms length (or thereabouts) so if you pixel peep and view the image at 100% (which may be 5 feet across on many computers) from a similar distance, then the DOF has been massively reduced and the faces at 12 and 18 feet will look horribly out of focus.