How to calculate custom fee during transaction?

I need to calculate custom fee at runtime, according to apigee documentation following scenario is happening

custom attribute number * rate = charge to developer (Current Scenario)

But I need to add custom attribute value and rate defined in rate card. The difference is that I need to add two values but default functionality is performing multiplication.
custom attribute number + rate = charge to developer (Requirement)

How can I achieve this?

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