It would probably help to know how you plan to process the film, and how you will meter the light. Since you plan to do this right away, my suggestion would be to just go ahead and expose the film at EI400. It's a good solid tolerant film and should print up well. You could even probably use the "sunny 16" type of exposure rule, or some variation depending on the light. If you want to use this fine film some more, and do your own processing, I strongly suggest that you determine what Exposure Index works most comfortably for you. A fine place to start is:(posted 8805 days ago)http://www.wenet.net/~halfhill/speed1.html
Best of luck tomorrow; enjoy the run. Get in the spirit of things, and you can take some great pictures. It's not the time to worry about the film. If I got stuck with a mechanical camera and a dead light meter, in the bright sun I would put it on f/16, 1/500 sec. and leave it. In dimmer light, open up. With a camera with a good meter, trust it for now and don't worry. Study the results later. Have some fun!