It takes a given amount of force to hold up your X. It doesn't matter which spring you use to do it, they will all be setting there holding up so many pounds. With 137 lb springs it will take longer for the spring to ramp up to that given value. Thus the reason coil-overs travel so much better. The one calculator above is kinda neat being a person can choose how much % of spring one wants to use at static height. The hard part is finding out how much the truck weighs minus the axles, tires, etc. (unsprung weight).