A question I am frequently asked by eclipse photographers is what exposure setting works best. The answer to that question is not easy as it depends on a number of factors - some you can control and others you cannot.
a) Optical system: The optical system is your camera plus the lens (or telescope). You can obviously control this aspect of your photography mission. There are tools online for estimating exposures. These are based on the focal ratio of the optics and sensitivity of the camera to light. Focal ratio is simply the focal length of the optical system divided by the aperture (front lens size). The lower the focal ratio, the faster the system. The faster the system, the shorter time required for an exposure. Light sensitivity in cameras is measured in terms of 'speed'. This speed value is often expressed using the acronym ASA or ISO (standards organizations). The higher the speed, the faster the light can be recorded.
b) Weather: You can't control the weather but you can work with it. If the sky is clear and the Sun is bright then you can get details of the solar surface. Light clouds or haze will block a lot of sunlight making some filters almost useless in terms of seeing solar surface details such as sunspots. Of course, during a total solar eclipse you want few or no clouds anywhere near the Sun and Moon! A clear blue sky is best - just not always achievable.
PRACTICE! PRACTICE! PRACTICE!
To figure out the best camera settings, practice with the system before the eclipse (or transit) event. Try a variety of exposures and try them under different weather conditions. Pretty soon you will develop and almost intuitive feel for the "right" exposure setting.
For the transit of Venus and annular eclipse, the pace is comfortable. You can shoot a variety of exposures and work with them later on the computer. A total solar eclipse on the other hand is almost frantic when it comes to photography. You cannot really practice the stress condition. But you can practice enough to become confident in your skills (for totality practice, take pictures of the Moon).
Just make sure the camera and lens are focused. This is probably the hardest part. So here are some tricks I've picked up over the years.
a) Use a dark hood. Using a small blanket or towel over your head that covers the camera makes the view in the finder stand out better. You can see finer detail and achieve a better focus.
b) Mark the focuser. If you are using a telescope this is very handy. Most manual lenses have approximate focus distances marked on the lens. In the case of a telescope we are focusing at infinity so only a single mark is needed to indicate about where that sweet spot is located. Use a light colored nail polish or permanent marker to indicate where the focus point is located. THEN ADJUST based on the weather and optical condition (normally a very small adjustment is all that is needed).
c) If you wear glasses, or worse, bifocals - use the longer distance side to focus the image when looking through the camera. Believe it or not, this takes a bit of practice and you will find yourself looking through the close up side if you are not careful.
And remember, at the end of the eclipse, you can always trade images with others should yours not come out great or only show a portion of the features visible.