Retargeting is limited by the audience, not the budget
Prospecting can absorb almost any budget; retargeting cannot. Once every reachable visitor has seen the ad as often as makes sense, more money only raises frequency. That is why the plan starts from traffic: the audience sets the ceiling, and the budget follows from it. If the budget this calculator returns is small, the constraint is traffic — and the fix is upstream, in the reach that fills the audience.
Why match rates keep falling
Consent banners, shorter cookie lifetimes and browser tracking protection all shrink pixel-based audiences. The durable answer is first-party data: customer lists and hashed emails you send to the platform directly, which match on the person rather than the browser. That is the difference between an audience that erodes every month and one that grows with your customer base.
Reading the return side honestly
Retargeting takes credit easily. Many of the people it reaches were coming back anyway, so platform-reported ROAS on retargeting almost always overstates what the spend caused. Treat the return estimate here as an upper bound, and test it: hold out part of the audience for a few weeks and compare. Check the result in the ROAS calculator against your break-even line, not against a platform benchmark.