Taken straight from my good friend who's an engineer.
If you go through and calculate the deflection of a coil spring under load and take a ratio comparing it to the load you get the spring rate. This will be in units of force/length. If you want to see the derivation you can pick up a copy of Shigley's Mechanical Engineering Design Handbook or a similar text.
The equation for spring rate turns out to be:
d is the diameter of the wire, G is the shear modulus (a material property of the metal that you usually look up on a place like matweb), D is the diameter of the spring and N is the number of effective coils (more on that later)
As you can see changing the number of coils not only changes the length of the spring but also the spring rate. This can lead to a rough ride or poor handling. The shocks, which from the factory are matched to the springs will also no longer be matched to said springs.
But wait there's more!
If your factory springs have different coils on the ends to make them sit flat in the perches, cutting also changes the N from earlier. You can go through calculations...or use this table:
The number you want to use for N is Na
As you can see changing the ends can effectively change the coil count by as much as 2 coils.
Now how do you translate this to ride height? The answer is fairly complicated and requires solving simultaneous equations involving trig and/or doing some calculus.
If you are doing a relatively small drop you can do some approximations, but these depend on the pre-load on the spring and suspension geometry. Luckily people sell software to do this for you. Or you can do what everyone else does and chop off little pieces at a time until you get where you want. Or you could not be a cheap ass and buy proper springs.