When it comes to scale, your best measuring stick will be against your most recent campaign results.
Higher spend means higher CPM’s because you’re bids are competing against more advertisers for your target audience. Higher CPM’s typically mean higher CPA’s, but higher CPA’s at scale is where you can really generate revenue.
Don’t let larger ROAS metrics from 3 to 6+ months ago stain your confidence moving toward scale. Performance goals should be mostly determined by what has happened in the account in the last 30 days. Anything beyond that is more or less navel-gazing.