To use this feature, please keep two things in mind. First, you have to assign the extra credit work to the "Extra Credit" category you selected and, second, the max points for each extra credit assignment must be greater than zero.
In order for a student to earn the maximum boost you've set (say 3% for this example), they have to earn 100% of the extra credit work assigned. If you make two extra credit assignments worth 10 and 15 points and a student earns 8 and 12 point respectively, their boost will be 2.4% (20 points earned / 25 points possible = 80%. Then, 80% of 3% max = 2.4%).
If the extra credit assignment is worth zero points for a max, then no percent average can be calculated (division by zero error for you math teachers).
This particular way of giving extra credit is different than other methods, such as adding on extra points to an existing score or giving points on an assignment worth zero points. When using the extra credit category with weighted grades, students must do all the extra credit work to keep the maximum percentage boost. A student who stops doing extra credit assignments after getting 100% on the first one will see their extra credit boost drop over time. Initially, they earned 10/10 but did not do any of the next 3 extra credit assignments (each worth 10 points), so now their average is 10/40 and they only get 25% of the max boost.