There is no such UI rule. This is done automatically.
There are two relationships defined for Priority. Priority vs Impact and Duration vs Priority.
The impact vs priority is defined on Administrator Console/Data/Codes/Service Level Agreement/Service Level. For each service level you can define how Impact is Related to Priority.
When you change the Impact on a service call automatically OVSD retrieves the associated Priority (if there is no SLA, the Default Service Level is used).
The Duration vs Priority is defined on Data/Codes/Service Call/Service Call Priority Duration Settings. There, for each priority you define the maximum duration and from there the deadline is calculated.