Unfortunately, it requires "brute force." Also, the coordinates you enter might depend on the type of site/system you are entering. For example, for a single-site city, you might enter coordinates/range to just cover the geopolitical area (even though the system can be heard from farther out) where the coordinates would be the center of the area rather than the tower location. For a multisite statewide system, better to use the site tower locations and a radius.
For site locations, you can click on a site in rrdb, then click on the license and you'll get a map of the tower site(s).
I actually go even further in subdividing for my local area...I create seperate multi-site systems for each patrol district, then create coverage for a patrol district using multiple overlapping "sites" that approximate the patrol assignment area. So, when I am in West Arlington I'm not listening to North Arlington calls. Required a lot of time with the mapping software plotting all this out.