You can set a minimum ROAS for your Facebook ads.
First, make sure that you're running a conversions campaign and select your pixel and a conversion event that utilizes value.
Second, you need to have Value turned on when configuring your web events within Aggregated Event Measurement. This takes up at least four of your 8 slots.
Third, you'll optimize for Value instead of Conversions within Optimization & Delivery.
Finally, you can set your Goal ROAS. Facebook will use this as a guide rather than generating the most value. In other words, consideration will be made for both the value that's returned and the amount you've spent on your ads.
Like all strategies, this isn't for everyone and shouldn't be used all the time. Like the Cost Cap and Bid Cap strategies, it's something to try. In this case, you'd experiment with it when you're generating conversions but not the ROAS that you desire.
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