CGrateS - passing account's balance to GrantedUnit (4g)

Question

I am using the latest version of CGrateS for my billing system and using open5gs for core system. When updating sessions we reply with a GrantedUnit which is some constant. For example, imagine an account has 1GB of data in its balance. When assigning 1MB GrantedUnit in configs, Every time 1MB is over core will request extra Units. But I think it is not correct because of the load it creates over the network.

How can I set GratedUnit to account's balance? And is it even a good idea to this?

here is the config file I used in CGrateS:

request_fields:

{"tag": "Usage", "path": "*cgreq.Usage", "type": "*constant", "value": "5242848"}

rely_fields:

{"tag": "GrantedUnits", "path": "*rep.Multiple-Services-Credit-Control.Granted-Service-Unit.CC-Total-Octets", "type": "*composed", "value": "~*cgreq.Usage"}