The lowest position would be 1. As you go higher up, you are compressing that spring resulting in a stiffer ride. If you go too high, it will jump back to the number 1 mark which would be the softest position. But at your weight, my guess it you would sit on the bike and have the shock compress, but not return to the original height of the ride.
Technically, the way I've dialed in the shocks on previous rides is to set them at a position and then sit on the ride. You want the shock to compress, but then return to the original position. This will give you the most travel at your given weight. If it doesn't compress at all when you sit on it or slightly bounce then it is too stiff. And so on.