Once I had constrained both controllers, I grouped them in a group called 'ClavicleGroup', which would make it easier to manage these controllers in the outliner, and also if I wanted to keyframe both controllers simultaneously.
I set a maximum value of 10, a minimum value of -10 (unlike the tutorial which states to set it at 0) and a default value of 0. When the fist is set to 10, it would severely clench, whereas when the driver is set to -10, the fingers would curl back, towards the knuckles.
As well as a fist attribute, I created a new attribute called Spread, which ranges from -5 to 5. When set in the positive direction, the fingers spread out wide, whereas if they are set in the negative, they narrow. Here is a video of my finger drivers in action.