How to use default formula in the rate definition for the mentioned requirement?

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Requirement: The rate definition value needs to default based on an employee’s grade and overall salary (Total). For instance, if the grade is A, the rate definition should default to 20% of overall salary (A different rate definition already configured) ; if the grade is B, it should default to 30% of overall salary. (image.png).

Note: Overall salary is also a rate definition and is part of the same salary basis in which the required rate is needed

Question / request for help: In the Rate Definitions screen (see screenshot), there appears to be an option to set a default using