Persistant, first off, thank you for supporting FitDay by being a Premium member!
What I usually do is have most of my custom foods set up as weight, because I aslo use a food scale. I have gotten used to "eyeballing" it when I can't weigh it. I have a good idea what the weight or serving size is.
Some people also put a note in the title of the custom food. Lets say you have almonds as a custom food, you may know that 30 grams = a serving and usually 30 almonds = a serving. You could set it us as a weighted (if that's the correct word) item but have the title say "Almonds 30/serving"
That's just a suggestion, there may be an easier way of doing it though.