You do not 'have' to take in 500 if the hcg is working. Don't worry so much about going under 500 just make sure to not go over!! You are mainly living off of your fat stores, so keep that in mind. If you skimp anywhere make sure it is not the protein, never skimp on that.
Hope that helps, I've lost over 100 pounds and most of the time never even ate the entire 500.
Best wishes on continued sliding!!