First off, there's rarely a situation where you should be starting off a new Google Ad Campaign with Maximize Conversions Target CPA Bid Strategy. Typically it's better to start off a new campaign with Manual CPC or Max Clicks Bid Strategy until it's generating about 20-30 conversions.

Now assuming you have a campaign performing as such, the next question that arises when you're testing out Maximize Conversions Target CPA Bid Strategy is what to set your Target CPA at?

What I'll typically due 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. The Target CPA I set is based on what the campaign has been averaging to date. If the campaign has been averaging a $100 CPA for example, I'll typically start around there when testing out the Maximize Conversions Target CPA Bid Strategy.

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.