It's submarine science.Reg servicing isn't rocket science.
The best thing you can do as you disassemble a reg for the first time is to take copious amounts of pictures. Wear that button out. You can always delete them later. As for orings, there are just a few sizes used, and the difference is so large that it's hard to make a mistake. However, it's important to learn to discern which o-rings are static (tiny lube), which are dynamic (a bit more lube), and which require a lot of lube (spools on SPGs).Assuming they did it right the hardest part seems to be making sure you install the right oring into the right spot.
Let me amend this a bit. There are a few things that many shops don't do that help you accomplish a great rebuild. Check your IP and cracking pressure before you open that reg. Take a pic of the first stage with all the hoses attached.