GOT IT!
I had to put the Rule on the AVX Sales Ledger BO.
IF AVX_Sales_Ledger IS NEW OR AVX_SalesLedger WAS CHANGED Then
SystemSettings.TotalAVXCreditDue=SUM AVX_Sales_Ledger.CreditAvailable WHERE (AVX_Sales_Ledger.ReportedToAVX IS UNDEFINED OR AVX_Sales_Ledger.ReportedToAVX='No')
Then in the Query of the AVX Sales Ledger Credit I set the Refresh properties to refresh when AVX_Sale_Ledger BO was changed.
Watching the log I can see the Rule Execute to update the SystemSetting.TotalCreditDue. I have the box ticked to run the process calc Credit Due after saving the AVX Sales Ledger Form but the process never ran, not sure why?
Looks like don't need to call the process anyway as I have the same SUM expression as a Rule on the AVX Sales Ledger BO.
Not sure if this was the best way to do this but it is working 😃
Roger