Last summer, my appetite for quantifying my finances intersected with my food-procurement habits to yield a math exercise: How much of my Amazon Prime membership was I chipping away with these discounts at Whole Foods?

The Seattle retail leviathan’s 2017 purchase of the Austin-based grocery chain consolidated a large portion of my annual consumer spend at one company. It also gave me a new set of benefits for the Amazon Prime membership my wife and I have had since 2011: an extra 10% off sale items except beer and wine, plus some Prime-only deals.

(Personal-finance FYI: Amazon also touts getting 5% cash back at Whole Foods on its credit card, but the American Express Blue Cash Preferred offers 6% back on all grocery stores. That higher rate combined with Amex Offers for rebates at designated merchants easily erases the card’s $95 annual fee and returns more money than I’d get from Amazon’s card.)

So on my way out of Whole Foods, I created a new Google Docs spreadsheet on my phone and jotted down the Prime savings called out on my receipt. Then I did the same thing after subsequent visits. If Whole Foods and Amazon were going to track my shopping habits (which I assume they could from seeing the same credit card even if I didn’t scan in the QR code in the Amazon app at the checkout), I ought to do likewise.

Aside from $10-and-change savings during last July’s Prime Day promotion and again on roses for Valentine’s Day, most of these 41 transactions yielded $4 or less in Prime discounts. But after a year, they added up to $118.14, just 86 cents less than the $119 Prime annual fee.

To answer the obvious question: No, I did not step up my Whole Foods visits because of this tie-in. That place does happen to be the closest almost-full-spectrum grocery store to my home, but there’s a Trader Joe’s barely further away that trades a smaller selection for cheaper pricing on staples like milk and flour. And thanks to this dorky habit of mine, I can tell that I’ve shifted more of my business from WF to TJ’s the past few months.