How do I create a function to give a value to a calculation

Example:

Total number of errors 3
Number of clients signed 3
% errors to clients signed 100%

If the error rate is 0% the formula should calculate 100%
If the error rate is 0%<21% calculation should be 80%
If the error rate is 20%<41% calculation should be 60%
If the error rate is 40%<61% calculation should be 40%
If the error rate is 60%<80% calculation should be 20%
If the error rate is =/80% calculation should be 0%

I have tried IF but just get errors (still learing all these formulas)
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Andrew