To determine which is the nearest Zscaler DCs for the specific IP or machine. Here are some commands that you can use to achieve that:
You can run the below command in your command prompt to see the nearest DCs for specific IP.
Replace the IP with your public source IP:
Or, you can run the below command directly to get the 3 nearest DCs for the machine on which it is running:
Once you gotten the best 3 DCs, can copy the IPs from the output and match it with the config page.