To calculate your monthly budget, you need to know what your daily average budget will be. So, you should calculate what you are comfortable spending on a per-day basis. A PPC management agency will optimize your campaign spend according to your clicks and conversions. However, even though your campaign spend may vary, you will not be expected to pay more than your daily average budget or your monthly spending limit. You can use tools such as Google Ads’ Performance Planner to help you optimize your budgets for all of your existing campaigns.