Welcome. Returns are generally speaking a matter of comfort and subjectivity on most pieces. Also depending on the strapping method you plan to use for the torso, the returns will vary. A good rule of thumb IMO is 3-5mm on biceps and forearms, erring on the thinner side. Shins and thighs, about 5mm is good to start but as you fit it and adjust to your body you may want to thin that or even eliminate such as behind your knees.
The torso (ab, chest, kidney, butt) I would say 5-7 for the outer returns , but many people like them thinner for aesthetic reasons. For the edges between plates, somewhere around 10-13 is good if you are using strapping brackets, so you have space for the screw heads. You can go less if using snap plates.
Remember these are rules of thumb. Proportion and fit to your body is the ultimate goal. Really study as many images from the screen used armor in the reference section of the forum.
Other people's builds are helpful, but may have inaccuracies you aren't aware of. So don't just go blindly by what you see there. Just because someone else got approved with it, doesn't mean it's correct.
Good luck and remember to go slow. Savor the build, you will be finished before you know it.