I hope you have an automated way of pulling the tariff percentages.

Seems like it changes hour to hour.

Fun! That being said, I think that the extension is very inaccurate because it can't know how much the product was actually paid for when imported from China (and therefore tariffed).

I would argue the 40% estimate, even though probably correct some of the time, is very often vastly overestimated. Consumers mostly pay for last-mile delivery, marketing, and the retailer's margin. The price paid in bulk to manufacturers is usually a very small fraction of the retail price.

Note that tariff categories can be pretty tricky. For example, a board game might be imported as a toy (125%) or as a table or parlor game (20%). Both categories are valid choices, and it simply depends on which the importer specified on the paperwork.