Mapping of scores to a field value in atlassian jira-urgent

Hi Team,

I have list values(values are string format) in a field called A and also a field called ‘B’ similar to A. Now i need to map the ‘A’ & ‘B’ field value with some numbers. Ater mapping i may have a field(may be ‘c’) to calculate and arrive at some value.

Say for example…

|field A| Mapping|

|Field B|Mapping|
|design issue|1|
|configuration |2|

|Field C|average of field A and B|

please let me know how can i achieve this.
this is for a service desk project.

Nte: I have tried issue score plugin(did not work for mapping)
I have scriptrunner also installed.

Someone can me help here urgently please.