You're right; it's a little wacky. I'm not sure what the default is, but when I do custom foods, I put the serving size (oz., cups, 1/12 recipe, etc.) in parenthesis next to the name. Then I enter the values that correspond to that amount. If I eat a different size, I just adjust the serving when I log it in (.5, .75, etc.).
Hope that helps!
And now that you don't have to be perfect, you can be good.