I just like to have only the eyes in focus in the face, so subject-background-seperation will not help here. From Zeiss I only own the 135 f/2, and I wish to get it tag sharp at least 90% of the time at f/2 without zooming the live view on the camera.
You can easily calculate the distance to the subject by using
D = F * H / h where F = focal length (in your case 135mm), H = subject height or width ( I'll guess 450 mm width) and h = sensor height or width (if you're in portrait orientation it would be 24 mm width. From this, I get your subject distance is going to be about 2500 mm.
Now go to www.dofmaster.com
and do your own calculations. I got that the total DoF will be 3.9 cm split nearly equally between front and back. That assumes the circle of confusion is 0.03 mm. If you want the image to be a fair reflection of your lens' capabilities, you can set the CoC at 0.015 mm and you'll find the total depth of field is 1,9 cm. That's 1 cm in front and 1 cm behind. If you're human, you will sway by several mm (as will your model - even if he / she is trying to stand absolutely still.
The result is that even if your focus is perfect, you might only get 50% of your images sharp but you should get about 25% sharp. This is why it's far smarter to stop down the lens a little and make sure you nail every image.
It's easy to test this - focus on something that's difficult to see then shoot off several images. You can even use this to estimate how much you sway which should inform how fast an aperture you should be using.