I think "IP 28 would levy a 2.5 percent tax directly on a firm’s sales in Oregon that exceeded $25 million. But the legislative agency’s calculations were based on a lower, or “equalized” tax rate applied to all businesses — regardless of whether they would pay increased taxes directly under IP 28." is important. 

The report pretended everyone would be taxed, but at a lower rate than the 2.5%. I don't know why a report would start with such a simplification and then expect the answer to be anything close to what would actually happen. I'd be interested to see a report that just looked at the 50 companies paying 51% of the tax. It sounds like those are pretty easy to determine, and the impacts could be even manually curated for better precision.