I came up with a little Excel spread sheet
that will automatically calculate the amount per serving of your recipes. Just enter the number of servings the recipe makes (lower left green box), enter your ingredients across the top in row one, then enter the nutritional values of each ingredient down each column. The spreadsheet automatically adds everything up and divides it by the number of servings you entered. You can then go and enter this info to create a custom food. If you save the spreadsheet under a different name, you can have it on hand if you decide to modify the recipe in the future.
Here's the Link. When prompted, save it somewhere you'll remember where it is.