If I’m choosing a server location to reduce ping, I start by thinking about where the majority of players physically live, not where I live. That sounds obvious, but I’ve seen groups pick a server near the admin because it feels “right,” and then half the players suffer. What I do is make a quick list of everyone’s region and pick a location that minimizes the worst ping, not necessarily the average. For example, if I have players split between the UK and the US East Coast, I usually test UK, US East, and sometimes a midpoint option, then pick the one that keeps everyone under a reasonable threshold.

My hands-on method is simple: I ask two or three players in different areas to run a ping test to the hosting city if that info is available, or we just spin up a short trial server and check in-game latency at the same time of day we actually play. I also keep in mind that routing can be weird. I’ve had a server “closer” on the map perform worse because of the internet path our ISPs took. In addressing this issue, I found that sites like this are useful.
Just to add, once the location is chosen, I try to reduce extra lag by keeping view distance and max zombies reasonable so the server isn’t creating delay even when ping is low.