In Mathspace, the index of 100 is given to a student who can prove full mastery.
Teachers can change the mastery score a student needs to reach for a given subtopic by changing "Target Mastery" at the bottom of a page when you create an assignment.
To do this, simply select the existing task, then select Edit Task.
From there you can set a new Target mastery.
Note that the Due date must be in the future for you to be able to adjust the Target mastery. The Due date can be edited first. The updated target mastery will appear for your students right away!