Useable Wheel Travel is the total vertical distance that the suspension allows your wheels to move without binding. If the lower coilover mount is welded to the axle, then this measurement is that same as the shock stroke.
Coilover Angle is the angle at which the coilover is mounted, measured in degrees from absolute vertical in any direction.
Lower Arm Length is the length of the lower A-arm, trailing arm, or 4-link bar measured from the frame pivot to the wheel center.
Shock Mount Distance is the measurement from the frame side pivot of the lower link to the lower coilover mounting point. If the lower coilover mount is welded to the axle, then this measurement is equal to the lower arm length.
Sitting Droop Ratio is the percentage of the shock shaft positioned inside the shock body at ride height. (For example, if you want 6" of shaft showing at ride height on a 14" coilover, that's 8" of shaft inside the shock: 8/14 = 57%)
Corner Sprung Weight is the total weight supported by the coilover. Sprung weight does not include axle or wheel weights.
Dual Rate Spring Calculation Guide
Performance off-road coilover shocks are designed to have two different springs installed on them such that the vehicle transitions from the softer spring to the stiffer spring at some point during the compression stroke. For 8" and smaller coilovers, there might only be space for a single spring while 16" and 18" coilovers may require a third "tender" spring to prevent the springs from going loose at full extension.
In a dual rate coilover setup, the lighter, primary spring is placed on top of the usually stiffer, secondary spring. While both springs will compress together, although at different rates, the upper primary spring is the effective spring rate until it is fully compressed or physically stopped by a transition nut (secondary nut) on the coilover shock body. Only after the primary spring is fully compressed or stopped, does the secondary spring come into play.
In general, your primary spring rate should be as close to your target spring rate (as calculated above) as possible. The secondary spring should be about 25% to 50% stiffer than the primary spring. Third rate tender springs are often around 50 lbs/in since their sole purpose is to keep the lower springs pressed against the lower spring plate at all times.
Triple Rate / Tender Springs / Helper Springs
Tender springs, also known as helper springs, are used in applications were the primary springs don't allow the coilover to settle down low enough under the weight of the vehicle to reach the desired ride height. This happens most often with long travel coilovers in applications with a relatively low ride height. Tender springs are low rate (50 to 90 lbs./in.), short (3" to 5"), and often made with flat wire. They remain compressed under load until the other springs fully extend at which point the helper spring expands keeping a load on the springs to prevent them from going loose.
Video: Coilover Springs, Spring Rates, and Calculations