Kerry MacDonald, University of British Columbia Head Coach, explains his 70-30 rule about drills to gauge whether they are the right amount of difficulty. He explains that for an attacking drill, he still dedicates 30% of that drill to the lead-up steps to attacking in real game situations: the serve, the pass and the set. He also never does drills that allow his players to succeed more than 70% of the time; "growth comes through the failure," meaning failure is necessary in drills for players to improve.
Skill development: The 70:30 rule
Volleyball Canada Chief Sports Officer, former Head Coach of UBC Men's Volleyball