Exposure Calculator window

To determine the exposure time, i.e. shutter speed, you have to open the Exposure Calculator window in Mercury Venus Transit Maestro.

Choose Camera > Exposure Calculator…
To display the suggested time exposure for one of the Mercury or Venus transit events, select the event, the sensitivity (ISO), the lens aperture (f/number), the Sun’s altitude and the elevation of the observer’s location. The time exposure will then be displayed and computed every time you modify one of those parameters.
Don’t forget to input the correct values for the Sun’s height, in degrees, and the elevation of the observer’s location, in meters. Those two parameters have an influence particularly when the Sun is low.

The time exposures are only given as a guide. They will help you choosing a film matching your equipment and the event you are shooting. An automatic sequence can also be build accordingly.
However, on the day of the transit, you should bracket your exposures one or more stops to take into account the actual sky conditions and the variable nature of these phenomena. If the sky is not clear or hazy, then lengthen the exposures.