You don't need to measure your impact. It's enough to describe it. "Helped a large client improving their web presence for achieving increased conversion rates." Of course, you were part of a team and of course, you can't show precise statistics. But if they existed, would those stats be worth anything to a recruiter? Your next job will be different in many aspects. No recruiter will read some stats and say, "oh, that person's work increased the company's revenue by 1.5%, so we expect the same result at our company."

In a CV, the impact you made is a story to tell, not a number to calculate.