IMU GPS Fusion

I think I read somewhere that Panda and All-in-one Boards do the fusion of IMU and GPS via the Teensy and so the IMU GPS Fusion should be 100% one or the other. I can’t find where I might have got that from. Default is 70:30. Is there a setting to use when using PANDA sentences, or should I just fiddle with it until I get it going pretty right?

I have a bit of yaw jitter that I had adjusted out in the pre-Teensy days. The IMU heading looks very steady, so I assume I need more IMU, but do I need 100%?