422 calculate ratio estimation

[giuliag92]

Summary

This pull request calculates the ratio estimation treshold.

Checklists

This pull request meets the following requirements:

• [x ] installable with all dependencies recorded
• [x ] runs without error
• [x ] follows PEP8 and project specific conventions
• [x ] appropriate use of comments, for example no descriptive comments
• [x ] functions documented using Numpy style docstings
• [x ] assumptions and decisions log considered and updated if appropriate
• [x ] unit tests have been updated to cover essential functionality for a reasonable range of inputs and conditions
• other forms of testing such as end-to-end and user-interface testing have been considered and updated as required
• [x ] tests suite passes (locally as a minimum)
• peer reviewed with review recorded

If you feel some of these conditions do not apply for this pull request, please
add a comment to explain why.

[sarahcollyer]

All looks good!