Install a third-party tracking system that will measure how well your digital marketing is going. This enables you to just focus on the insights and improving your digital marketing.
Then work backward from the digital marketing strategy, what is our aim here? How do we measure that? Then link this to your metrics.
For example, if your aim is to attract new customers, what is a meaningful amount of potential customers? Then use your measurement to see how many people you have attracted.
That is a simplistic example but is how you build up, proving out the success. We invested in digital marketing to deliver X and we got these results.