My car was approximately 80 to 90 feet from the corner. A prohibited zone extends from the beginning of a sign until the next regulatory sign or the end of the block. If there is a sign in the middle of the block and no other regulatory sign, then the prohibited zone extends to the end of the block. This is so notwithstanding the length of the bus or busses that use the area. If a sign is missing and the missing sign ends the bus stop area, that presents an additional problem.
Lets discuss the possibility of an extra large bus stop and only one sign regulating it.
You must take photos of the sign and the rest of the block indicating where you were parked. The photos should be specific enough to allow the judge to make sure that the area shown is in fact the area summonsed and how far away the sign actually was. If the sign is missing you would have to show a photo of the missing sign, also with specificity to show the location vis a vis the summons. While we are on the subject of long bus stops, let's discuss yellow lines. Yellow lines are usually not an official indication of any kind.