There are at least a couple major problems with this method. First, it's pretty easy to overlook quite a few expenses. Things like that daily stop at the coffee shop, or the cost of a car registration that happens only once a year.
Be Realistic: Set achievable goals for saving and stick to it. Constricting expenditure seems difficult in the beginning but slowly habit sets in. As a motivation factor, annual household budget templates that provide estimation of how your budget will function for the entire year.