How to Tune for RTK

PX4 Autopilot
1

I am using RTK and expecting to get centimeter accuracy. In my GPS_RAW_INT message I see that my h_acc is 1.5m

I see on the PX4 documentation about rtk tuning: RTK GNSS (GPS) | PX4 Guide (main)

that the default parameters are tuned assuming a GPS accuracy in the order of meters. It gives the example that one could decrease the EKF2_GP_V_Noise and EKF2_GPS_P_Noise to 0.2 - but that is just an example right? How do I know what is the correct value for my setup? Am I supposed to measure some noise value (if so where)? Do I take some value from the spec sheet of my RTK?

Thanks!