I think it's a "ballpark" as you mention, specially following what David has said before. I deducted 1/2 coil from the bottom side of the spring only, cause that end came from the factory with a closer than the rest, loop or turn? On the top, I didn't deducted the 1/2 coil, cause that end was cut by me and the space between that end and the next coil is the same as the space between the rest of the coils. IMO Yup, "ballpark" figure.You would loose a half a coil on each end when installed due to bottoming of the first coil. Cutting the spring may not allow bottoming and would change the way you figure rate.