What is the best way to make sure my campaigns don't go over a monthly budget?

For example I have a budget of 1200 for July. My ads are running everyday, but at the weekend only about an average of 5 is spent. Weekdays gain a lot more impressions/CTR.

I know the theory is to do 1200 divided by 30.4 to get a daily budget, however is this the most effective way to set the budget for the month?

Thanks