RTL adjustement

Hello everyone

I am using Ardupilot for scientific plane. For my first time i have a communication failure for the telemetry.
So the plane was very hight in the sky ( 500 m ) but the rtl altitude was 100m. So i see the plane doing a beautiful dive. Taking to much speed and loos the wing when it try to stabilize at 100 m…
I am completely lost with multiple parameters to adjust ?
is it possible to find a tuto for this point ?