Targeting Cook County buys you Schaumburg and Berwyn whether you meant to or not
The county line is the laziest geographic target available here and it funds markets you may have no intention of serving.
Cook contains the whole city and a long list of suburbs including Evanston, Skokie, Oak Park, Berwyn and Schaumburg. Selecting the county selects all of it. A firm whose real catchment is the North Side plus a few near suburbs will spend a meaningful share of budget on clicks from thirty miles out.
Build the targets from where clients actually come from and where you actually appear. Pull the last hundred matters, map them, and let that shape the polygons. Radius circles drawn from your office address are a shortcut that ignores how people here move.
Keep DuPage, Lake, Will and Kane as their own campaigns if you serve them at all. They are separate auctions with separate competitive sets, and folding them into a Chicagoland budget means the loudest market eats the rest.
Set location options to presence rather than presence or interest. Otherwise you pay for people researching Chicago divorce law from another state, and family law attracts a lot of that.