What I've always done in the past is to use FitDay, but go forward to a "new" day and use it as a dummy of sorts. I put in the individual ingredients in the quantity used for the recipe. Once I have it all in there, I jot down the numbers of the totals (cals, fats, protein, carbs...). Then I create a new custom food, name it, and put in the correct numbers based on serving size. For instance, I make something I call pumpkin pie-less (basically just pumpkin pie without the crust) and the recipe I created makes 8 servings. So I just divide everything by 8 to get the numbers to create a new food.
Trust me, it sounds more complicated than it really is.