how do I determine the specific instanceId for a given report
Its quite simple to get the Instance ID for a report through its URL. Consider an example of Contribution Summary report, the url for it is civicrm/report/instance/7?force=1&amp;reset=1 So the instance id in this case will 7 i.e integer value anything after civicrm/report/instance/ will be your instance id or you can use api explorer to get the instance ID.

HTH

Pradeep