How many of you use a ridgeline to control the sag of your hammock? I suppose it would also allow you to control your height off the ground. Currently, I use a ridgeline only for my tarp assembly but I do not use one for my hammock.
Does it provide a better lie. Secondly, is this ridgeline in addition to the one you use for your tarp? Lastly, is there a formula for how long your ridgeline should be?

My setup is very simple right now: tree straps, whoopie sling, ridgeline for the tarp with prusik knots to adjust.

I use an adjustable structural ridge line. It is made from a smaller whoopie sling. I tried many times on the length until I got it right, then tied it so it cannot move. This way I have the same lay every time I set it up. Edit, it does make for a better lay you just have to try different lengths. Try it with your hammock at home with a wratchet strap until you get it the size you want. Then just make a solid ridge line cord.

I made a UCR ridge line for my latest hammock. I'll more than likely leave it adjustable as it will be the one I lend to friends & family. But once I find the perfect length for me I'll just make a mark on the amsteel with a sharpie so I can get back.
A ridge line length of 83% of your hammock length is a good starting point.
The ridge line of you hammock is not the ridge line for your tarp. They should be separate lines.
Hammockfourms is a great resource for hangers.
And this hang calculator is kinda slick. I have the iPhone version that I use quite a bit.

I don't use a ridge line for my hammock, I do use one to get the bug net on my hammock up and out of my face, and I use the same ridge line to hang my tarp