I don't know if this solution would work for you, but this is what I would do:
I would make a private recipe for the bread (not shared). Once it's baked and cooled and I'd weighed weighed the entire loaf, I would enter the total number of servings in the calculator as equal to the # of grams (or ounces, whichever you use). For example, if the loaf weighed 475 grams I'd enter that it made 475 "servings."
Then, when I weigh each individual slice, I can enter the number of grams in the "servings" field. If the slice weighed 40 grams, I'd enter 40 servings and the calculator would show the correct number of calories.
I hope this helps!