Wolfram Alpha added the ability to calculate nutritional value three years ago. By combining several ingredients like this: "748g flour+508g water+1.87g yeast+12.342g salt", the nutrition facts are generated.
blog.wolframalpha.com/2009/08/07/calculating-your-daily-food-intake-with-wolframalpha/
wolframalpha.com/input/?i=748g+flour%2B508g+water%2B1.87g+yeast%2B12.342g+salt
For example, 150 g slice.
These don't work:
I couldn't figure out how to get wolfram to calculate value per slice. It is possible though to just take the ratio of each slice instead of using the whole formula. So a 150g slice is about 3/25 or .12 of the whole loaf. In front of each ingredient multiply the ratio. For example, "748*.12g flour".
748*.12g flour+508*.12g water+1.87*.12g yeast+12.34*.12g salt
Wolfram is a bit finnicky about the syntax so I think its easier to reduce the weights to one slice first.