What is the best way to get tack sharp portraits with shallow depth of field when lacking an auto focus point where I want to focus. I don't want to crop and "focus and recompose" is not an option since the shallow depth of field will throw the person out of focus. Of course a precise measurement of the distance is possible, but also very cumbersome and to do manual focus is a bit of a guessing game. Any ideas or do I have to relax my requisites?
Your best option is probably to put the camera on a tripod and use magnified Live View to do precision manual focusing.
Some cameras support focus peaking via an electronic viewfinder or the Live View screen, which allows the camera to tell you what parts of the entire frame are most in focus. This may or may not be helpful when you are trying to isolate your subject using selective focus at a narrow aperture. Focus peaking will show what areas are in sharpest contrast, but won't necessarily tell you if what is in sharpest contrast is perfectly focused.