Hi Paul
Hopefully, someone else will give you a better answer, but, here is a suggestion.
I don't have an image handy to test this idea. First, put a best guess in for the focal length. Take a single shot, with plenty of stars, where in the sky doesn't matter and it doesn't need to a particularly 'clean' image.
Upload it to this
site, when it is analysised, the site will give you plenty of info about the stars in your image. It will also tell you what the angular size is. If you are lucky, the focal length will be there as well. Or at least enough information that it can be calculated.
Good luck
Philip