Welcome, I am just down the road (I-5) from you...mid Willamette Valley. There is an excellent soap supplier in Portland...Shay and Company. Their hours are a bit of a PITA, but I have been very happy with the quality of their ingredients. Just picked up some Cocoa and Shea Butters yesterday morning on my way to work. They don't carry Micas and have a limited selection of FOs/EOs, but I'm fine with that. I also get my Olive, Coconut and Avocado Oils from Costco.
I've never actually measured but Google says 6 teaspoons to an ounce...which isn't correct because I just measured and I got 28 level teaspoons. LOL
Here's the thing...it doesn't matter because whatever the recommended usage, your mileage is going to vary depending on your recipe, any fragrance or essential oil you use, the colorant itself and the color you are looking for. And in addition to Micas, you have Clays, Oxides and Pigments that are also sold by the ounce and/or gram. As an example...I made a pound of soap the other day and wanted a 'baby blue'. I don't have any 'baby blue' Micas so .I used just maybe a quarter teaspoon of Electric Blue Mica and then added TD. I made a Chocolate Espresso soap...the FO naturally discolors to a medium brown, but I wanted a darker, more even shade of brown so I added a tablespoon of Hershey's Cocoa and a teaspoon of Toffielious Mica to 2lbs of oils and got a really rich shade of brown.