Spark will create this deficit for you. If you input your current weight and how much fitness you plan to do (you can list it in calories, which would be best for you, so multiply your burned calories by 4 or 5 for days of the week when you burn that much) and it'll tell you how many calories you should be eating.
You might not be eating enough for the amount of fitness you're doing, so I definitely recommend using Spark's tools to figure out what it should be. If you love numbers, you can use their formula and figure it out for yourself; here's the link explaining it all: www.sparkpeople.com/resource/calorie_calcu
Good luck reaching your goals!