Your chance to gain with each use depends on your chance to successfully use the ability. You will get the fastest gains when your success/fail rate is around 50%. As you get closer to 100% success rate your gains will slow down. (The ability isnt considered challenging enough for you to learn from anymore.)
This means that for shadow jump you will get your best gains around 62.5 ninjitsu (50% chance for shadow jump). As you get closer to 87.5 ninjitsu (100% chance for shadow jump) your gains will slow down more and more. Once you hit the low 80s you will mainly get gains from your daily 0.1 GSG. After 87.5 you will not get any gains at all from shadow jump