Since a coil spring is a torsion bar wound in a circle, you can estimate the change to the spring rate since the wire material, diameter, and coil diameter stay the same, only the length changes. If you estimate the existing spring length and estimate the new spring length, the ratio of the length is also the ratio of the change in spring rate. The spring rate on a torsion spring is proportional to length. And...you can simplify it even more....if you have eight coils and you cut one off, you make the thing 14% stiffer (8 divided by 7).....ouch. Two coils make it 33% stiffer. WOW. Get that thing down by cutting 3 coils from the eight.....60% stiffer.

The earlier post about sneaking up on it is correct. IF you torch the springs, you change the spring material properties and it becomes more of a guess.