If you use a Custom Spend (for example to include taxes on top of a platform's spend), and you're about to create a Custom Conversion whose CPA, ROAS or other cost-based metrics should be built on that Custom Spend, follow this order so those metrics are right from the start.
1. Set up your Custom Spend first
Create and publish your Custom Spend before creating any Custom Conversion that depends on it.
2. Create your Custom Conversion without generating its cost-based metrics
When creating the Custom Conversion, you'll reach the step where you select which calculated metrics to generate (CPA, ROAS, COS, AOV...). Skip the ones you'll want to base on your Custom Spend (you're required to select at least one metric to continue, so keep just one and rename it, e.g. "Not used" to mark it as a placeholder you won't add to your reports).
3. Build your cost-based metrics. as Calculated Metrics
Create your real CPA, ROAS, COS and any other cost-based metric yourself, using the Calculated Metrics module, with your Custom Spend directly in the formula. For example:
CPA = [Your Custom Spend] / [Conversions]
ROAS = [Conversion Value] / [Your Custom Spend]
COS = [Your Custom Spend] / [Conversion Value]
Rename them as you wish so you can recognize them easily in your reports. Once created, these behave exactly like any other metric and can be added to your reports and dashboards.
When to do this
This is a one-time configuration step, not something you need to repeat on an ongoing basis. Follow this order once, when setting up your workspace, and your reports will stay correct going forward. You'll only need to do it again if you create a new Custom Spend or a new Custom Conversion later on.