Hiya Jocelyn,
A negative balance would do your application a lot better than the current surplus you have - this indicates to them that you could save up or pay off the surgery.
If things vary month to month, give them your biggest month.
Okay, try and remember all your expenses there may be lots you have forgotten.
Gifts, (if you added up the costs of all Birthday, Christmas, Wedding, Anniversary gifts and divide by 12) Donations, take away meals, daily coffees, art unions, insurance, Doctor's Bills, Medications, Vitamins and Supplements, Dentist, Public Transport, Parking Fees, Road Tolls, Dining Out, DVD Rentals, Hairdressers, Nails, Dry Cleaning, Church, Health Insurance, Holiday Savings, Cinema etc.
Hope this answered your question or helps at least.
Cheers
Tara