I'm looking to use a piece of paracord as a ridge/sag line in my hammock setup, to do this I want to be able to tie the two ends together in a fashion that will allow me to alter the length of the loop easily but to lock in place - this knot will be taking my weight and my gears overnight so it needs to be solid.
I'm not sure how your setup is, but I'd try a Purcell prussik or something similar.
You must test its holding Power on paracord yourself, but on climbing cord it's solid enough.
There are multiple ways to do what you're looking for. One way I solve this problem is with a metal ring. You attach a fixed knot on the ring, pass one end of the rope through one loop of the hammock, wrap the rope 3-4 times through the ring and then a fixed knot on the other loop of the hammock. Slide the ring up and down the ridgeline to adjust the size.
Dutchgear has an image of this setup.
The type of knot you are looking for a called a friction hitch, or a slide and grip knot which is a kind of knot used to attach one rope to another in a way that is easily adjusted.
There are many different ways to tie a friction hitch:
And of course the Purcell Prusik, but the Purcell Prusik was developed for ascending ropes and doing rescue work, it may be a bit complicated to your purposes.