I would shoot a test roll (or at least a partial roll) shooting the same scenes at ISO 12, 25, 50 and 100. Shoot several images this way. If you can develop at home, you can cut the film and develop what you have shot and save the rest, if not, you can either shoot the whole roll with heavy bracketing or just guess and shoot the remaining portion of the film at 50 and send it to a lab. You can then pick which EI works best when viewing scans or prints. If none of them look good, consider developing them as B&W or tossing in the trash