I have two abs pieces that I would like to create a waterproof seal between via compressive force via fasteners like screws. How much do o-ring/gasket compress? Certainly it is dependent on the material and how much pressure is applied, but are there any resources on this? There is a pocket on one of the abs pieces that will be mating with the other piece on the bottom with very tight tolerances, as I plan to put something in the pocket and seal it with the second abs piece.
With any O-ring seal, static or dynamic, the key parameter is "O-ring Squeeze" which is defined as the percentage reduction in cross sectional area when compressed from the free state. Since the o-ring acts like a spring, this squeeze is directly related to the sealing force. The greater the squeeze, the greater the sealing force, and the greater resistance to leakage...at least in general.
As with most things, more is not always better, too much squeeze can result in a poor joint since it can cause excess stress in the mating parts, excess friction (in the case of a dynamic seal), and O ring damage since there's a greater risk of the o-ring getting pinched between the two mating surfaces.
In designing O-ring seals in stainless steel with good surface finish, I would try to stick to < 15% squeeze for dynamic seals and 15%-30% for static seals using a standard NBR O-ring from Parker, but it really depends on the type of O-Ring, the materials involved and all of the other specifics of the application.
There's a lot that goes into the design of a good O-ring joint. The best resource I've found is the Parker O-ring Handbook (link goes to the pdf).
The amount of o-ring compression or squeeze is dependent on the size of the ring and the specific application. The o-ring groove geometry likewise varies. The easiest way to design with o-rings is to use a design guide. I know it is a commercial website, but Apple Rubber provides a lot of nice design information for using O-Rings. For the record, I have no affiliation with Apple Rubber.
Here is a GENERAL guideline on O-Ring Gland design that you are describing: Oring Face Seal Guide
There are several resources online(google search)
Pressure, size, fluid type, serviceability, safety(redundancy) etc will influence your design. If you have more details, we could provide a more detailed answer, but this should get you started.