One of the most common pieces of dieting advice is to eat “everything in moderation.” But many people have a skewed sense of what “moderation” means, a new University of Georgia study suggests.

Researchers asked the study participants two questions: 1) How many cookies do you think you should eat in one sitting? 2) How many cookies would you consider to be a moderate serving?

Nearly 70 percent of the subjects defined a “moderate” serving as larger than what they thought they “should” eat. On average, people guessed you should eat only two cookies at a time—but that eating three cookies would pass for eating in moderation.

The researchers also surveyed the study participants about what kinds of junk food they preferred, and what they counted as a moderate portion size for each type of treat.

The more someone liked a food, the more generous they would be with their definition of a moderate serving size.

The upshot: People are misinterpreting advice to eat in moderation as a license to eat whatever they want, says study author Michelle vanDellen, Ph.D. It can give you a justification for poor eating habits, she says.

The problem is that moderation is, admittedly, a vague concept.

The logic behind it is sound, says Men’s Health Nutrition Advisor Alan Aragon, M.S. It’s a great way to avoid completely prohibiting certain foods, which research suggests may lead to disordered eating and weight gain.

But without any concrete guidelines, it’s hard to tell whether you’re overeating or not.

So here’s a specific guideline from Aragon: Only 10 to 20 percent of your total daily calories should come from junk food.

That will allow you to indulge—helping you to stick to your healthy eating plan in the long term—without overdoing it.

Find out how many calories you need in a day here. For example, if you’re a 40-year-old active guy, you need 2,800 calories, and 280 to 560 of those can come from whatever treats you want—no guilt necessary.