Sometimes, I fully calculate the recipe. I track everything in Excel. I have one tab for calculating recipes. So, one column is the item and the next columns are calories, fat and carbs. The bottom row is the total values. The next line is labeled something like "Serving 1/9" and has the total values divided by the number of servings. Then, on the tab where I track my intake, I enter the appropriate entry for my actual portion. If I had two slices and my "serving" was only one, I double it, etc. So, that's what I do when I feel like it.
When I don't feel so ambitious, I google for something similar. In general, I'm not going for a high level of accuracy here. It Google tells me my dinner was 450 calories and it was 550 or 350, that's OK by me. I make most of our meals and breads from scratch, so, after that, I don't always feel like doing the math!