Hi,
Thanks for reaching out. The way mentioned to calculate the balance is by subtracting the spending from the pre-set budget on the account. The Account Performance Report is the report for retrieving metrics for a specific account within a date range. Once you get the Cost value from the Account Performance Report, you could subtract it from the budget you set for the account for the same period of time to know how much budget left for the account.
Thanks and regards,
Xiaoming, Google Ads API Team
Hi,
There is a better way to get this value by fetching the account_budget.amount_served_micros field from the account_budget resource. You could refer to this documentation on how to write the report query.
Thanks and regards,
Xiaoming, Google Ads API Team