Ride height is all that matters, that is the basis for the spring calculation. Pre load is incidental to spring rate at ride height, it is not an independent adjustment. The lower the spring rate the higher the platform will need to be and the stiffer the spring rate the lower the platform will be. On your struts you should have about 3/4" of shock ram showing before it hits the bumper at ride height. If the spring were too light you will bottom out the shock when the platform is maxed out to it's highest adjustment. The amount of compression the spring has at ride height vr's it's free height is the springs pre load. The spring you asked about is a 300# spring that fits the description you gave for your strut.