For years, lip injections were a hit-or-miss procedure in beauty industry. If done incorrectly, people were often left sporting the dreaded “duck face” look that screamed, “I had work done.”

But now, those days of iffy lip looks can officially end thanks to a recent study that finally determined the exact amount of filler that lips can take before looking artificial or over-plumped.

According to the study conducted by JAMA Facial Plastic Surgery, 98 volunteers—76 females and 22 males with an average age of 42 years old—were used to evaluate photographs of a model’s lips in five different variations over the course of a 27-day period.

The upper lip, lower lip, upper and lower lips, and shape of the Cupid’s bow were all digitally altered in each picture and were later viewed by participants in an online survey. After viewing the images, each participant had to answer whether or not the lips appeared to have cosmetic enhancement, and if the lips appeared to be attractive and natural-looking.

Unsurprisingly, the results found that whether or not the lips looked artificial varied for each area. Half (50 percent) of participants thought that the lips appeared treated if the ratio of lower to upper lips was either less than 0.92 or greater than 1.48. On the other hand, 50 percent of responders believed that lips appeared treated if the ratio of lower to upper lips was either less than 0.85 or greater than 1.7.

Doctors also found that enhancing the upper lip alone had a higher chance of being perceived as an augmented area, whereas injecting both lips with filler had a much lower chance of coming across as “fake.”

Although, it’s important to keep in mind that this study proposed several limitations, most notably the fact that cultural and ethnic background can greatly influence a person’s aesthetic perception.

The takeaway? If you make an appointment for lip fillers, it’s best to treat both the upper and lower lip in order to look the most natural. An over-filled top lip is a clear sign of a lip augmentation, so be sure you go to a board-certified plastic surgeon, facial plastic surgeon or dermatologist that takes into account your lip proportions for the most natural outcome.