Stop looking at last click
Almost no one reads an article and buys immediately. They read it, remember the name, and search for you days later. Attribution models that credit only the final touch will always show PR as worthless.
Look at branded search impressions in Search Console before and after coverage. That is the cleanest available proxy.
The three numbers to watch
Branded search volume, direct traffic, and the conversion rate of your non-brand channels. Effective PR lifts all three, because more of the market arrives pre-warmed.
Set a baseline before the campaign starts. Without it you are guessing.
The honest limitation
You will not get a clean revenue figure per placement, and anyone offering one is modelling rather than measuring. Judge PR on whether the market knows you better than it did last quarter.