TL;DR

A quick forecast estimated page-one SEO gains at 1.1 extra clicks a month in one case and 0.5 in another. That small estimate can still help decide whether to ship.

how much traffic could a page-one SEO gain bring?

In one case, the estimate was 1.1 extra clicks a month. In another, it was 0.5.

Those are tiny forecasts. They still put a number on the possible traffic upside before investing.

would you still ship for less than two extra clicks a month?

That is the decision the forecast makes easier to face. The estimate is small, and it gives you a concrete basis for asking whether the work is worth shipping.

A tiny forecast can be useful even when it does not promise much traffic.

FAQ
How can I estimate SEO traffic upside before investing?

Estimate the likely extra clicks per month before committing. In these cases, the estimates were 1.1 and 0.5.

Is a small SEO traffic forecast useful?

It can help you make a shipping decision. The forecast is small, but it makes the expected upside explicit.