That is a great set up! I wouldn't worry about a little cable stretch. Just double check that the combined weight of both of those plates should be half the weight of your finished gantry. If those are 45# plates that means your total weight of your finished gantry would be 180#'s???? Seems very heavy. Also, while you've got it set up, you might want to adjust your weight up or down in 5# increments and autotune again and save the configuration file for in the future if you change your gantry and add or remove mass, you'll have another configuration file ready to go that you could upload without having to autotune again.