Yes, you can easily adjust the doneness of the beef according to your preference. The cooking time we have in the recipe should make the meat about medium. But, if you prefer medium well done or well done, you can add an additional minute or two to the cooking time for each side.  You can also use a meat thermometer to double-check how done your burgers are. Between 120 and 130 degrees, the burger is rare. 140-150 is medium, and 160 and above is well done.