For those who are curious. I use this rigging setup quite a bit, while it is not always the best option for every horse and every saddle it comes in handy. With a wide, low-withered horse it helps pull the weight of the saddle evenly over the ribs and keeps it from sliding over the shoulder, it also reduces saddle roll (when you mount or dismount). On a slab-sided (or flat-sided) horse (where the barrel and the shoulder are pretty much on the same plane with no real contour between) and most mules, it keeps the girth from galling (making sores) and also helps keep the saddle from sliding onto the shoulder.
My preferred way was to have a very long latigo, to start AND finish at the rear rigging D'. But I have found that a shorter latigo will work, you can start it just like normal at the front rigging D', go through the cinch, up to the flank rigging, and then buckle off at the cinch. If the latigo is long enough, I would take it back up to the flank D' and tie off a latigo knot. In the pictures below, with Gibson's big belly and a short cinch, I have to settle for just buckling at the cinch and poking the slack through the keeper. The key is that you do not want to 'finish' your centerfire rigging on the front rigging D', it needs to have the final tightening and fastening pull it BACK toward the flank. If it is 'finished' by being pulled through the front rigging and buckled or tied in a latigo, the saddle and girth will still pull into the 'heart' or forehand of the horse as you ride.
Center-fire rigging has to be done on both sides, and you will find you can adjust how far forward or back the cinch goes, so be sure it is the same on both sides.
I recommend practicing it just for fun, even if you don't use it you should know how in case you are out on the trails and you get a cinch-galled horse. The only problem is you need 2 latigos, an off billet won't work. I have been resourceful in the past and swapped an off billet with a latigo on another horse I was riding with; so one horse's saddle was held on with 2 billets and I center-fired the cinch-galled horse.