The center ridge is a common thing is a lot of gathered end hammocks. Sometimes it can be eliminated by making a wider hammock, so you can lay on a steeper diagonal. Sometimes it can be eliminated by making the ridgeline shorter so it sags more. Sometimes it's as simple as increasing the angles on your suspension lines, and hanging the hammock loosely enough that the ridgeline doesn't have very much tension in it.
Worst case scenario, a lot of people like to put a pillow or rolled up jacket under their knees to help correct for the calf pressure.