One finesse is to raise the hammock (Bridge or GE) foot end about 6 to 10 inches higher than the head end. It doesn't cause a "head down" situation, but it does prevent sliding towards the hammock's foot end during the night. Warbonnet recommends an even more aggressive head/foot height difference.
A small camp pillow is often used, not for your head, but as a prop to place under a knee or both knees. For me, if the pillow has any bulk, it would tip my head too much if used there. But if I use the camp pillow for my head (back sleeping), the filling in the middle can be distributed to the sides so there is very little lift under my head and the support comes from the sides (primarily to keep the bug net further from my face). But if you are on your side, I understand you want your head/neck aligned.
For me, the Bridge is firm enough at its recommended hang angle (25° for a WarBonnet RidgeRunner). The tighter it's hung, the more initial "tippyness" is felt when you first get in. But it is really stable once you are in it and settled down. Just remember to put a hand on each edge as you enter/exit. A friend of mine prefers a steeper angle, giving it more of a bathtub feeling (secure sides).
A little more on that "foot end higher" ... Imagine if you will (Twilight Zone theme playing in the background), a nice curve representing a hammock with both ends the same height. As you move away from the Center, there is an equal upward angle in each direction. We know it really doesn't happen like that because you are on a diagonal, but just imagine. Now, as you lower the head end and raise the foot end. The part of the hammock towards the foot end becomes steeper, while the part towards the head end becomes flatter. Because you are on a diagonal, your feet don't "go up", but the hammock feels even flatter. Now all that is determined by hammock size (length and width) and camper size (height and weight). You may not feel any difference at all, or it might be a game-changer for you.
There is no formula, no magic exact number to achieve, for "perfect" comfort. There is also no substitute for taking your hammock to a park - somewhere there are two good trees, and setting it up, measuring heights/distances, trying slight changes, and finding your individual sweet spot. Then you can measure the distance between hammock ends when hung, and the hang angle, and the distance above the ground, so you can duplicate it on your next hang (see previous posts about a hammock Ridgeline). If that park (test site) has a second pair of trees, see if you can duplicate the setup on them.
Note that you can use various body parts as "rulers". For example, how high the hammock suspension is relative to head height. Or using outstretched arms and hiking poles to measure tree distance. Remember, the further apart the trees, the higher your tree connection, in order to maintain the same hang angle as with trees closer together.
