It's quite easy. Just remember that you are dealing in thousanths of an inch, and a little change can change the pattern. Just the wrong torque on a bolt, or a tiny bit of runout, can change the pattern slightly.
As for holding the third, I just took two pieces of square tube, and welded a tab to it that extends about 1.5" off the side of the square tube. I then drilled a hole for a 8MM bolt.
I bolt the third onto the tabs, lift the whole thing onto the workbench, and clamp into place.
As far as your question
Part of what intimidates me is not understanding how things like pre-load/backlash/etc effects driving on the road/trail.
Preload determines two things. More preload will reduce gear deflection, which is a good thing, because when gears deflect, the pattern changes to a less favorable one, which causes the gears to break. Too much preload though, will make the bearings fail.
For the pinion, on new bearings, you don't want to exceed 20 in lbs. For used, I wouldn't exceed 10 inlb. For used bearings you just have to determine how used they are. The idea behind preload is that when the bearings are at the end of their usable lifespan, the bearings are still under a bit of tension so that things don't wobble around.
For the carrier, on a 8.4, IMHO you cannot get enough preload on the bearings. To get a decent ammount of preload, you have to tap the shims in. One shim higher and you can't shove it in. Same thing for these bearings, no more than 20in lb for new.
You can measure pinion preload easily, but not carrier bearing preload easily. To measure CBPL, you have to install the carrier with no gear. Then somehow spin the carrier with a wrench attached, to read the figure. With a 8.4, just get it as tight as you can, you won't get about 20in lb...
For a 8", Zuk already calculated it. For 10 in lb, you need roughly 100 ft lb torque on the sidewheels. For 17in lb, you would ahve to torque around 150 ft lb on the sidewheels.
As far as depth goes, I always have this argument with other gear installers. Zuk sets them up favoring the deep side of the ring gear, as do I. Why? The pinion teeth are stronger. So by going deeper on the ring, means you will be shallower on the pinion. Which ok, they're stronger anyways. Also, when the gears deflect from being under load, the pattern will work towards an even balance anyways.
The trick is to not go too deep that you take too much strength away from the pinion, and to also keep things quiet.
Too tight of a backlash means that when the gearset is running, oil cannot get between the teeth, resulting in more wear. I feel tighter backlash leads to a stronger setup, especially if the pattern is ballanced between coast and drive, and between the heel and toe, but favoring the heel. A loose backlash will generally make the difference between goast and drive greater.
It's a balance. Get the pattern balanced as close as you can, but not burning up the gears. I don't go tighter than .005"...