Here's a good article on pie dough thickness https://www.insider.com/how-to-make-sure-your-pie-crust-is-the-right-thickness-2016-11, though I've generally used two pennies as a thickness guide than two quarters.
Two quarters are 3.5 mm thick, two pennies are 3.04 mm thick.
An eight of an inch is 3.175 mm, which is in between the two, but if you stacked a dime on a quarter it would be 3.10 mm thick.
I think I actually prefer the all-butter pie dough I make to be a little thinner than 1/8 inch. I find if I use my 1/8 inch silicone pie dough thickness guides, pressing down on them results in a dough that is just a bit thinner than 1/8 inch, because the silicone is flexible. King Arthur Baking used to have maple thickness guides but that's just one of the many products they've dropped from the catalog.
I discussed this in my blog post on measuring pie dough thickness: https://mynebraskakitchen.com/wordpress/measuring-pie-dough-thickness/.
I still haven't figure out how to get a pie dough (OR ANYTHING ELSE!) uniformly thick using a tapered rolling pin, and when I took the pastry course at SFBI we used rolling pins that were not tapered.