You can play around with the recipe, less or more eggs, less or more milk, using only yolks, change some or all the milk for cream, adding seasonings (cinnamon is quite popular)
You can soak the bread for as long as you want, play around and see what you like best
I use about a tablespoon per side of the bread but you can add more or less depending on the side of the pan, you can also spread the butter on the outer side of the sandwich to get a more even color