Art was one of Leonardo da Vinci's greatest tools. It allowed him to be able to break down problems into a visual form. It is an essential skill required to be a well-rounded generalist.

Composition

Lighting and Shading

Proportion

Perspective

Color Theory

Depending on what you're aiming for, each of these may require more practice than others to achieve the desired 80%. If you're aiming for the sketches Leonardo makes, I'd recommend prioritizing Composition, Lighting and Shading, Proportions and Perspective as your main focus.

Overall, hone these 5 core concepts. Practice them through drawing, rather than theory-crafting.