You can:(posted 8425 days ago)1. Open up (stop down) the lens 1/3 stop from the 400 speed to represent 1/3 stop slower (faster) film, i.e., 320 (500).
OR
2. Change the film speed on the meter and re-read. If light hasn't changed, you should get the same net exposure. If you don't your meter isn't linear, and needs fixing or replacing.