ACOS only compares spend to the sales your ads get credited for. TACOS compares it to everything you sold, ad-driven and organic combined, and it has the same profitability ceiling break-even ACOS does.
Why TACOS has a ceiling too. TACOS and break-even ACOS aren't different math, just a different denominator. ACOS uses ad-attributed sales; TACOS uses total sales. But the royalty per copy doesn't change depending on how the sale happened, so the same break-even percentage caps both. If TACOS is above it, ads are costing more than you're keeping across the whole book, not just the ad-attributed slice.
There's still no universal target. The ceiling above is specific to this book's price and format. A launch push can mean running close to it on purpose for a while. What matters is knowing where the line actually is for this book, not guessing.
Tracking this by hand, book by book, every week, is exactly the kind of thing that stops happening once life gets busy. Troven Ads Optimizer tracks TACOS and break-even automatically, per book, and finds the weekly changes that keep both moving the right direction.
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