Setup Process Noise Covariance of robot_localization with enabled ~use_control parameter

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Hi, I am unsure how to set the Process Noise Covariance when I enable the ~use_control parameter.

I have IMU & Odometry as measurements. I only receive absolute X,Y,Yaw from Odometry and Yaw and angular_velocity from the IMU. Therefore I use the differential parameter for odom so that the yaw measurements from the IMU actually have an impact on the ekf position.

two_d_mode: true 
odom0_config: [true, true, false,
           false, false, true,
           false,  false,  false,
           false, false, false,
           false, false, false]
odom0_differential: true

imu0_config: [false, false, false,
          false, false, true,
          false, false, false,
          false, false, true,
          false, false, false]
imu0_differential:false
use_control: true
control_config: [true, false, false, false, false, true]

For the prediction I enabled the ~use_control parameter, because I want the prediction based on the /cmd_vel topic (and not IMU linear acceleration).

I have done some experiments myself and calculated the X-Variance,Y-Variance and Yaw-Variance for the process noise. Those values are in the order of 10^-4.

My question is, if it is sufficient to put those three values into the process noise covariance matrix for X,Y - and yaw-variance, and leave the rest of the values unchanged.

The prediction step is based on the vx and vyaw (setup in ~control_config) and integrates that over time to get absolute position. So I would like to know if I need to setup the vx-variance and vyaw-variance in process noise covariance too, or if the variances are only used after the integration to fuse with measurement data.

Thanks in advance, Lascho