Over the week, you want to have your 7-day average hit the middle of your range.
So if you're at the low end on one day, and the high end on another day, it averages out. Typically, the middle of your range is effective for weight loss, but it's best to think about hitting the target over a week or a month rather than each day.
Slow weight loss (1-2 lbs per week) is more effective and easier to maintain over time. Extremely rapid weight loss generally tends to make people quit and regain (sometimes more than they lost in the first place). Think of this more as a real lifestyle change than a diet, where this is something you'll keep doing for life.