When cutting a male thread the blank is always made about 0.1mm/0.004" smaller than the nominal size. If the thread is really small than you might only reduce it by 0.002 and if it is a big thread than you might make it 0.008". This is to allow for wear on the tips of the thread cutting tool and makes sure that there is clearance between the crest of the male thread and the root of the female thread. The same process is also used when boring out the hole in preparation for the female thread although to a lesser degree. A properly fitted thread locates on the flanks and not on the ID/OD. Hence why your Nominal thread of M28 actually measures 27.94mm.

If you are using a tap and die as opposed to cutting the thread with a single point cutter such as in a lather or mill, then depending on the material and the thread pitch you may have to increase the clearance to stop the thread tearing or spinning the job.

On some tougher materials and/or multiple lots if you want to speed up the process but only need a commercial thread as opposed to a fitted thread you can screw cut to 80% depth and then finish with a tap or die at high speed and feed in one pass.

Hope some of this helps.

Matt.