Does it say 88% beef still?
Personally as you dont have the nutritional info I would just syn it as a standard burger. Its only a few syns and a one-off isnt going to make any difference.
Beef, burgers, standard, grilled, each 5
Beef, burgers, aberdeen angus, grilled, each 11
Beef, burgers, luxury/premium, grilled, each 4½