Regardless of which usmle step you took, to get your percentile, you need to base it on the mean and standard deviation. Look at what your score report says the mean and standard deviation for your test are. On my version of step 2, the mean was a 216 with a standard deviation of 23. As for whether or not a 243 is impressive on step 2, i have heard that step 2 is easy to pass but hard to do well on. Therefore, your step 2 score is pretty good. However, different programs may weigh the step 2 score differently.
Good Luck.