Child support
The best place to start for calculators and information on child support is here https://www.gov.uk/child-maintenance/overview and from there are some good resources and links.
The Henshaw Report evaluated child support maintenance and the report is available on the Department of Work and Pensions’ website and is worth reading to understand the problems which led to the formation of the CMEC: https://www.gov.uk/government/publications/recovering-child-support-routes-to-responsibility