A Ducati bulletin for 4-valve engines specifies that recommended tension for installing fresh-out-of-the-box new belts and the acceptable range of deviation (105–115 Hz). It also states that if you are going to reinstall a set of used belts they should be adjusted to 85–105 Hz. It further implies that used installed belts can be left alone if they measure above 70 Hz.

What Ducati is saying is to first set the tension of a new belt to spec, expect them to lose a slight amount of tension after some initial run time, and then verify at any later time that this tension is above the minimum requirement. In other words, you should not re-adjust the tension to the new belt spec 115 Hz max after run-in.
These values assume that you are using OEM belts made by Gates.