What's the best daily caffeine intake in terms of BENEFIT / HEALTH RATIO?

I'm wondering about something a bit beyond the simple "what's the maximum dose not to exceed?". We all know that caffeine, consumed wisely, has a very favorable benefit/risk ratio: improved focus, energy boost, better athletic performance, and even some suggested long- term health benefits from studies.

So my question is: for a healthy adult male, what would be the ideal quantity of caffeine in milligrams (mg) per day to maximize benefits (focus, energy, well-being) while minimizing downsides (anxiety, poor sleep, jitters)?

I'm not looking for the dose to pull an all-nighter or break a record, but really the "cruising" dose for optimal daily health and productivity.