The only thing I can tell you about tuning is that it pretty much a trial-error thing.
Knowing how to pitchbend, play with vibratos, even playing with lengths of notes or using extra sounds like ending vowels, breaths, etc.
All of these are what make your tuning something unique. I can recommend you to study some of the really nice usts around; like Aster's (Person in charge of Kyou Hana) and Chase's (Person in charge of Yonagine Kazuko and Toby Osbourne, Chihiro?)