You'd perform, 6 to 8 total 1.5 reps.. Then you continue the set by performing mechanical partials for another 5 to 8 reps that focus on the top end of the range of motion where you only lower the weight about one third of the way down. (nicktumminello.com)
If you don't fully extend your arms at the end of the range of motion of the movement, meaning at the lowest point of the curl, you are selling yourself short. (fitnessandpower.com)
The closer you are to parallel to the floor at the end of the range of motion, the more muscle work you'll feel. (ab-core-and-stomach-exercises.com)