It's not a simple calculation.
Spring rates are based on the diameter of the "wire", diameter of the coil, and the number of active coils.
LINK to Engineer's Edge
Spring rubber supports and couples the motion of adjacent coils. It doesn't completely deactivate a coil, but it reduces its contribution to the active coil count. Less effective coils = stiffer overall rate.
You can see in the table below that it can add 5-10% to the effective spring rate.
LINK to Hot Rod Magazine article
For the record, you can do this very safely. It's better to run the right spring... but you won't damage anything, even with multiple coils coupled.
