The truth is, no one is good at their job on the first day, What sets super achievers apart from middling performers is the speed at which they learn. So how do you maximize how fast you get better at whatever it is you're doing?

There are tons of tips out there on how to learn more quickly and efficiently, but Auren Hoffman, the current CEO of SafeGraph and a serial entrepreneur who has built five companies, says one rule stands above all others. When someone asked "What is the best thing a CEO could teach an entry level employee?" on question-and-answer site Quora recently, his answer was a principle that could be called "the 70-percent rule."

If it's easy, you're not learning.

Growth, we've all been told, happens when you push yourself beyond your comfort zone. That's true whether you're straining to lift a bigger weight at the gym or struggling to master a tough but valuable new skill at work. But we also know that there is such a thing as pushing yourself too hard and tearing a muscle or burning out psychologically.

So how hard should you be pushing yourself exactly to maximize personal growth without risking longer-term damage? Here's Hoffman's answer:

One way people, especially more junior employees, underestimate themselves is by failing to spend most of their time on things that are really hard for them to do. All employees (not just entry level employees) should strive to have at least 70 percent of their time doing things that are really difficult. These are the tasks that require the most thought, rigor, and attention. And these are the tasks that result in the most growth.

But, you might object, doing mind-numbing donkey work that teaches me nothing is a big part of my job. If that's entirely true, you should probably be aiming to find a new job. But in many cases, Hoffman says, employees who aren't stretching themselves haven't seized all available opportunities to offload routine tasks and take on more challenging work.

"Figuring out how NOT do something is almost always hard. Maybe you can have someone else in the organization do it (where it would be hard for them). Maybe you can outsource it. Maybe you can automate it. Maybe you can find a software vendor that does most of it. Maybe the organization doesn't really need to do it. Figure it out," Hoffman says.

If you're feeling a bit bad reading this, as you're nowhere near hitting Hoffman's 70 percent target, you can at least feel better that you're in good company. "I personally struggle with this and constantly need to remind myself to focus on leverage," Hoffman confesses.

So don't expect filling up 70 percent of your day with tasks that stretch your capabilities to be easy. In fact, I'm not 100 percent sure it's a sustainable target in the long term (I'm personally certainly nowhere near hitting it at the moment). But as a reach goal that keeps you continually striving to maximize learning and do your best work, the 70 percent rule is a handy rule of thumb to keep in mind.

Are you hitting anything near Hoffman's target?