Ducati 999 Owner's Manual page 178

Minimum tread depth
Measure tread depth (S, fig. 76) at the point where tread
is most worn down. It should not be less than 2 mm and
anyway not below the legal limit.
Important
Visually inspect the tyres at regular intervals for
detecting cracks and cuts, especially on the side walls,
bulges or large spots that are indicative of internal
damage. Replace them if badly damaged.
Remove any stones or other foreign bodies caught in the
tread.
E
fig. 76
79