Measure the spring rate - weigh yourself, then find a way to stand on top of the spring with all your weight and have a helper measure the new height. I'd suggest a vertical (fixed) steel pipe up the center of the spring with a piece of 1" plywood slid down over the top big enough for your feet. Divide your weight by the amount it compressed... that is the spring rate. Should give you a better idea, since you can find tables of which springs have what spring rate in various places on the net.