Jutsu Mastery is a person's proficiency in techniques. The below list is a reference point that characters should follow when they are deciding their jutsu mastery. The numbers you choose to add to your character pages should seldom surpass the percentage tiers listed below. Although it is possible it is rare.
In short terms, match the percentages to your character's rank.
Academy Student[]
(E-Rank, Weak)
0% to 10% Mastery
Genin[]
(D-Rank, Average)
11% to 30% mastery
Chunin[]
(C-Rank, Advanced)
31% to 40%
Jonin[]
(B-Rank, Proficient)
41% to 50%
Kage[]
(A-Rank to S-Rank, Exceptional)
51% to 60%
S-Rank[]
(S-Rank, Expert)
61% to 80%
SS-Rank[]
(SS-Rank, Expert)
81% to 100%