That's not a bad idea. A picture says so much more than trying to explain something in words.
Okay here's Esther.d Driving our Jackson Pillock at the RHS.
Note the big buckle that holds the loop the shafts go through, it's in the middle of the pad, where it should be, the pad protects the horse from the buckle. There is a brass plate on the shaft where it runs through the tug, on this plate is a small peg that sticks out to the side, sometimes it's under the shaft, this is the tug stop and when the buckle is in the middle of the pad and the horse is pulling on the flat, this little peg should be hard up against the back of the tug loop.
The tips of the shaft come no father forward than this manly chest.
The shafts and the floor of the vehicle are level, and the shafts come roughly half way up the ponies sides. Jackson's are a tad high because he's such a short arse! But nothing dangerous.
Look under the shaft and you'll see a little loop just in front of his hip, nest to it is another loop on the side of the shaft. These are the breeching Ds which are your brakes. You choose whether you use the undershaft breeching D or the one on the side, in this case we have used the undershaft one as it keeps the clutter down on the shaft itself. Note the breeching, close in resting on the horses quarters, if he was pulling on the flat or uphill you should get no more than 4 fingers between him and the breeching.
The breeching seat is about mid way between the top of his tail and his hock, horizontal neither flopping down or riding up.
On this cart there is another D on the shaft nearer the pony's bum, just level with his tail. This one is for a FALSE breeching, you never attach the horse's harness to these. This is for a leather strap, on offset squares to be slung between the shafts. This is only for showing on lawns, or show drives on very flat land, you take off the breeching on the horse and rely on the false breeching slung between the shafts to stop the vehicle. Not advisable on this turnout as the pony is so small and the false breeching would not come in the optimum place.
You can see the collar height here, above the point of his shoulder, but not digging into his windpipe. Tug in the middle of the pad, tug stop hard against the tug.
Now, here, to perfect the fit of the harness the traces could do with being a hole shorter. The tug buckle has swung to the back edge of the pad, still within acceptable limits. But if we did make the traces shorter, we might have to let the breeching out a hole, it all has to balance itself up.