Use consistent input data for variable rate tests
Use consistent input data for variable rate tests

Where we can it makes sense to use the same input data for tests,
as this is more concise and removes any ambiguity about whether
the numbers chosen are subtly testing other aspects of the code.

I've added some comments to help explain how I calculated the new
results from the new test input data.

alphagov/notifications-api,alphagov/notifications-api