Typically, we recommend enabling InfluxDB metrics export for ~1 week (to cover peak times in your region), you can do it this way: https://fastnetmon.com/docs/influxdb_integration/
After that, you can make query to detect peak traffic for packets per second, bytes per second and flow per second metrics. Then you can multiply these value to 2-3 (depends on your capacity) and use as baseline.
Also, it's very good to known limits of your network. For example, if you know that router cannot handle more than 1M packets per second then you need to set threshold way before it. And most important thing is mount of spare capacity from your upstream. If you have only 10G of external capacity utilised up to 90% then your baseline should not exceed amount of your spare capacity.