Yes indeed, I don't know what the linearity of any change of spring rate will be as the spring is compressed - until the coils bind of course!The spring rate is set when it is manufactured, I had assumed the rate would remain around 250lbs for all practical purposes throughout the range of operating lengths of the spring.

The point of winding the spring seat up is to adjust the ride height on the car to make it level fore-and-aft, having no adjustment at the rear on my car.
This compensates for the spring compression due to the weight of the car. When I used higher rate springs the seat needed less adjustment because the spring was compressed to a lesser extent. Sorry I didn't explain that this was about ride height, not rate.