As the ISO setting relates to the internal meter, and your meter is useless if I understand what you are saying, just disregard it. Get a handheld meter and use the ISO scale on it. Generally, there are detents on the internal camera meter ISO scales. If there is a detent between the 250 and 500 then use it if the meter is good. Make sure you have some practice with the camera before attempting its use for the project to make sure the rest of the camera functions as it should, especially the shutter speeds.