Calculated field

[Giuseppe Piersantelli]

We are using ScriptRunner plugin in our Atlassian cloud account.

I need your help to implement a time tracking function. Below the use case.

We use a Jira service desk project to track and manage the change requests that an external supplier analyzes and develop. The process is quite easy. 

1. periodically, we buy from the supplier a bunch of, say, 30 working days which will be used by the supplier to work on our software;
2. the project's stakeholders create on the JIRA board a new issue which describes the CR's requirements;
3. the CR is received by the supplier which estimates the required effort to develop it, say 3 days; the supplier uses the JIRA default field estimated effort (in hours), so he reports 24 (3*8); The CR is in status "Pending"
4. our IT department evaluate the CR and the supplier's quotation; 
1. if it's not okay, the IT rejects the CR; the CR is in status "Rejected"
2. if it's okay, the IT approves the CR and authorizes the supplier to work on the CR; so the IT authorizes the supplier to consume 3 days (or 24 hours) from the initial 30 days bunch; as a consequence, the remaining amount of paid days will be 27; the CR is in status "Approved".
3. in any case we know that a specific CR will potentially consume 3 days from the initial 30 days bunch.

We need a system which, starting form a determined amount of paid days, helps us to automatically calculate the remaining paid days for all the CRs opened on the JIRA board, both in status "Pending" and "Approved".

Is scriptrunner the right plugin for this purpose? May you please help me?

Many thanks ,

Giuseppe