Adding voids underneath analog signal for improved impedance matching
The "best" way is to build a board and measure the impedance. Another "best" way is to simulate it with some form of EM simulation software. The "quick" way is to use a rule of thumb like 5 tracewidths on either side (for microstrip). So, if your trace is 10 mils, keep all copper on layer 2 at 50 mils or more away.

However, that looks more like coplanar waveguide than microstrip...