I will provide you a brief idea.
Create an email notification and use email script to write logic for this.
Query incident table in there to get count (totalIncCount) of resolved incident within a time duration(say td).
Now Query incident_sla table with following filters
1. group by incident number
2. taskslatable_stage=achieved
3. Created within duration td
get row count(resSlaCount) from above Query and then use you formula in there
resSlaCount *100 / totalIncCount
You could then trigger this email using Scheduled job after a time duration td.
In addition to above filters , you may need to include additional filter for incident_sla name because one incident may be related to multiple of SLAs and among them , one SLA stage ma y be achieved while others is breached.
