When setting a Target CPA for your Google Ad Campaign, It's good to look at what your campaign's average cost per conversion is over the lifetime of the campaign. I'll then just set the Target CPA to start somewhere around there.

What I'll typically do is create a Google Ad Experiment where I switch the campaign to Maximize Conversions Target CPA Bid Strategy and give it 50% of the ad budget to see how it performs.

You can then try to lower it gradually overtime to maximize your ROAS or you can identify dimensions that are wasting ad spend and just exclude them. What dimensions? Dimensions like keywords, audiences, demographics, locations, ad schedule and device. For example, let's say you notice ages 25-34 are just driving up cost but not converting, you can easily exclude that age group so you're no longer wasting money on their clicks.