Offset is just a small part of the equation
Offsets aren't very useful without a frame of reference, and in a lot of ways they're about as confusing as using "ratio" as the measurement of a tire's sidewall height.
The two important pieces of information you should be able to gleen from an offset is the front-spacing and the back-spacing. The offset itself is a useless number on its own, but you can calculate both front and back-spacing which are very important.
The only reason you should really care about back-spacing is to make sure it doesn't interfere with the mechanicals of your car. The beauty is that you should be able to figure out "how much more" can my car handle by putting a tape measure on the back of your existing wheels and figuring out what you're going to impact.
For instance if my current car has an 8" wheel with a +20ET and it looks like I can run another 1" of wheel on the inside I know the most back-spacing I'm going to want would be 147mm (8*25.4/2)+20ET+(1*25.4). I don't know what offset that is yet because you need to know the width of the wheel I want to run to calculate offset.
As a note, you multiply by 25.4 to convert inches to mm.
Calaculating front-spacing is important for two reasons. Interference and making sure you get the right "look". For the most part you want to make sure the front and rear wheels look parallel. It would suck for your rear wheels to be a narrower than your fronts.
A good piece of information about your car is the stagger of your hubs on your car. You want to know if your rear wheels are inset. My car is pretty extreme with a positive offset on the front and a negative offset on the rear. The OEM wheels were +23ET on a 7" wheel and -20ET on an 8" wheel. Using this information I can calculate that my rear mounting location is 55.7mm shallower than the front (or at least if I wanted to match the look of what the manufacturer originally decided).
I figured this out by calculating the front-spacing of the OEM wheels and subtracting so I know the difference. When I build/buy new wheels I want the same difference so that the outside edge of my wheels are somewhat parallel (I'm not trying to match the OEM position, just making sure the wheels are parallel).
If I had a set of 8" wheels with a +20ET for the front and I want a 10" wheel on my rear, I can calculate that my target offset would actually be -10ET. I'll either need to build a wheel to match, or I'll need to buy spacers to make it work.
(8*25.4/2)-20 = [my front-spacing of my front wheels] = 81.6
(10*25.4/2) - (81.6 + 55.7) = [my target offset] = -10.3
NOTE: the 81.6 is my front-spacing of the front wheels and the 55.7 is the difference between the front and rear mounting locations.
I can't figure out the size of the barrel or the lip I'll need though, because that depends on the difference between the mounting pad and the mounting face of the center section. If you had that number you could also figure out your barrel and lip sizes for 3 piece wheels.