As I was writing about how I determine the sales price of my notes, I realized that I had the tax rate at 15%. Why 15%? That is that value i was using for my stock spreadsheet. Short term gains are basically regular income. To be on the safe side, I should probably use 25%.
Of the notes I've sold, one had a -6% APY (because of tax). I think this one was a typo when entering the sales price.
The rest were over 16%. Many much over 16%.
So I made a mistake with the tax rate calculation, but because of my margins of safety, I'm still getting more than 16% APY overall.