There is no hard and fast rule because it depends on your upper body torso and arm length. It's important that your comfortable. And even more so for long events than for shorter road races. For long endurance rides a more upright position is less tiring though less aerodynamic. In my opinion it is better for endurance riders to be more upright and utilized the drop bars when a more aerodynamic position is needed.
Road racers usually want the handlebars one to three inches below seat height. For randoneering I would think about level or one inch above seat height would be good. So then you need to get a stem of the length that allows you to have your handle bars at about level or one inch above the seat height when the seat is adjusted properly. And the reach to the handlebars is comfortable.