Slope_0 represents center-stick sensitivity. The value is scaled such that it's equivalent to RC Rate.
Shape interpolates between a curve based on Super Rate, and a curve based on Expo while holding initial slope and Max deg/s constant.
OpenTX Expo refers to the expo curve on an Open TX-based tranmitter--typically zero..
Checking the Derivative-box plots the change in Slope versus stick position.
FlightOne/Raceflight conversion offers several methods minimizing error. The "average" error is minimized in Methods 1 & 2. The single-largest error (over full stick travel) is minimized in Method 3. The "Narrow" search differs from the "Wide" search in that it only considers rate profiles whose initial slope is within +/- 10%, ensuring center-stick feel is preserved. Method 2 would be my recommended approach. The approach used by RotorPilots & FlightOne is Method 3.