I thought of an easy way to temporarily convert your existing ridgeline to an adjustable ridgeline.
In the pictures below, the large green cord is your ridgleline and the small yellow cord is a loop with a Klemheist knotted onto your ridgeline. The carabiner is attached to your ridgeline via a Marlin hitch, with the carabiner acting as the spike.
Now you can slide the Klemheist knot along your ridgeline, and the more you slide it, the shorter the ridgeline gets. You will need to get out of your hammock to slide the knot because it will not slide with the tension of your weight on it.