Take the time to look at the foods you eat and figure out the serving size. When you can break a serving size down to 2 tbsp or 60 m&ms, you can enter that number in Spark People. Instead of entering "1" for the serving size, enter the actual number of product that makes up a serving. You can then enter a partial serving and let spark people calculate it for you!
For instance, did you know that a serving of dark chocolate m&m's is 60? I added the food as a favorite but didn't enter "1" for the serving, I entered "60" serving. Then, when I eat the m&m's but I only eat 5, I'll enter 5 on the serving and Spark People will calculate the calories etc for me!