The answer is staring you in the face: you already took a digital image of the scene. Check the exposure information for that image. To simplify your calculations, set the ISO on the digital camera to the same value as the film.

A digital rendition will actually have less latitude than the B&W negative you are planning to use. If you can see the shadow detail you want and the highlights are not blown out, your exposure can be the same as the digital exposure.