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Mastery Tokens give 0.1% of the maximum mastery pool XP to Mastery Pool of the corresponding skill when they are claimed. Mastery Tokens are earned randomly while training a noncombat skill. The drop chance for mastery tokens is 1 in [math]\displaystyle{ \frac{18\,500}{\text{Milestones unlocked for skill}} }[/math]. Each thing you unlock by leveling the skill counts as a milestone e.g. smithing steel platelegs is unlocked at level 40 and counts as one milestone, additionally reaching level 99 in a skill counts as one milestone. Wearing Clue Chasers Insignia increases the drop chance by changing it to 1 in [math]\displaystyle{ \frac{18\,500}{1.1 \times \text{Milestones unlocked for skill}} }[/math].
Note that for Fishing specifically, it is more beneficial to wear the Amulet of Fishing instead of the Clue Chasers Insignia. This is due to the Fishing interval reduction provided by the amulet, which increases the number of actions over time, and in turn results in a greater increase in the average number of mastery tokens obtained when compared to the Clue Chasers Insignia. In addition, the Amulet of Fishing also increases the rate at which fish (and therefore XP) is gained.
Mastery Token Chance at Level 99