A 300# spring will need to be compressed 1.33 x the 400# springs previously compressed distance. Since the your new 300# spring is 1" longer, that distance will be decreased by 1". Example 400# spring was previously compressed 2". To maintain the same height with the 300# spring. 400# X 2"= 800# that was being supported. Now with the 300# spring 2" X 1.33 = 2.66" -1" =1.66" so in this case the spring platform would actually need to be compressed less to maintain ride height (.66"). Ah but there's more to it. You need to use wheel rates instead of spring rates. These can be calculated from the formula in the diagram below. Then assuming the difference in wheel rate you can then figure the difference in the amount of compression distance of the 2 springs. But by my math up until 4" of compression at ride height the spring perch wouldn't be turned any higher with the 300# spring than the 400# spring. Are you sure that you have the rates of both springs correct?
Doug