There are two "philosophies" to determining the center point/range (and neither is wrong).
1) Centered on the antenna site with a range that matches the likely receivable area.
2) Centered on a geopolitical entity (i.e. a town, patrol district, etc) with a range that covers the geopolitical boundaries.
I use 1) for the Texas DPS system, which has statewide coverage. I use 2) for local PD (so that I only hear West Patrol when I am in the West Patrol area, for example). Since geopolitical entities are seldom perfect circles, I use multiple "sites" to "paint" the area with circles in order to narrow down the coverage.
I used Delorme to plot out the circles for each system or set of system. As to coverage for the type (1) philosophy, you seldom can get more than 25 miles reception from a vehicle, so that is what I generally used as a max, but if you wanted to get scientific someone will need to recommend some VHF/UHF propogation modeling software.