What is the percentage of cardiac output delivered to the heart?

About 4%: About 4% of the cardiac output goes to the heart muscle itself. That is the same either at rest or with exercise.

Cardiac output: The major organs get about 80% of the total co. The percent each gets varies depending on the dynamics going on. As the heart can vary its resistance markedly coronary blood flow can vary by a factor of 5 depending on heart rate and resistance so it isn't a constant percent of co.