Let's break this down to coil overs/non coil overs

Non coilover(stock oem type coil spring--what is the measurement of spring pocket at lower arm level(preferred ride ht) position--now take a spring with a known rate and free length--install and see where it settles---make adjustments from that point with either free length at the same rate or new rate at same free length or combo of both

Coil-over--check total suspension travel from bump stop to full extend to determine the length of coil over shock to give lower arms level at midpoint of shock travel--
Pick a reasonable length/rate spring and install and let suspension travel settle--adjust the spring for the suspension ht you want---spring should not coil bind at bottom out or be loose at full travel---adjust from there for the ride and shock valving---expect to go thru a lot of springs to get the best ride/handling