Yes, the design load is the weight per spring it is intended to carry at the load height(compressed amount). Rate is the amount that will compress the spring 1". For say a 200 lb/in spring if 400 lbs is placed on it it will compress 2".
I wouldn't cut them shorter than what you have now, they will settle some after a little use. Only thing you can do is trial and error, and I know iit is a pain, but no pain, no gain