The threshold is 10% of physical RAM. It’s not configurable.
Server v4 will send out automatic email notifications on low memory. Meanwhile, you can design a project that will monitor Server logs, identify low memory warnings and send out emails if any. Schedule this project to run every 1 hour or so.
Another approach, is to make a project with a PowerShell script to get all running processes, then filter only the Server process, check its memory consumption, and send out an email notification if it’s above the desired threshold. This topic can be of help: https://stackoverflow.com/questions/26552223/get-process-with-total-memory-usage