In automotive applications, preloading leads to premature failure, but in a machine tool application, some preloadi is needed to maintain accurate location. BUT, as the graph shows, excessive preload WILL shorten bearing life, so use the minimum advised by the bearing manufacturer.
The preload for a particular bearing, or set of bearings, may well be expressed in terms of linear measurement.
Even if not stated to be 0.00X" for a particular machine, it may be expressed in terms of a number of degrees of rotation of the adjusting nuts, such as "Zero end float, then tighten by 30 degrees", based upon the pitch of the adjusting thread.
So don't overdo the good work, unless your hobby is changing bearings!
Howard