Steering wheels have a ratio that determines the number of turns needed to reach maximum turn.
Your first objective is always going to be to figure out what that ratio is for the specific vehicle.
The second task is to apply that ratio to the proper axis of the bone you mapped the steering column to.
The third task is to apply the input as a way to increase or decrease the steering value.
Because you are working with a known ratio, I suggest letting the input work directly on the biggest value (the steering degrees in either radians or degrees), and multiplying it down with the ratio to apply the value for the actual steering.