I eat a big or small breakfast based on how hungry I am in the morning, and I track it. Then, still in the morning, I enter what I'm planning to have for dinner, and then I enter an evening snack. Lunch is my "wiggle room." I can go big or small, depending on how many calories I have left.
If I want a bigger lunch, I'll start cutting away at my evening snack. This way, I have a full dinner to look forward to, rather than eating too much at lunch and feeling like I'll have to starve during dinner, which is my favorite meal. But really, any meal can be the wiggle-room meal!