This is better worded than I could do, its from equinesite.com so is their work!
Now, let’s review general fitting of the curb chain. The angle of the shanks to the corners of the mouth should be 45 degrees when the reins are pulled. Only this angle is the most healthy and productive – any more and the bit does not act properly without excessive pulling of the rider’s hands, and any less and the bit is too strong. A properly adjusted curb bit and an experienced rider’s aids are a delicate tool of training – a improperly adjusted curb in inexperienced hands is a torture device for the horse.
Curb Hooks
I could not leave out the subject of the curb hooks. So many people do not pay any attention to them whatsoever. On the upper shanks of every curb bit there are two hooks - one on each side. They should face forward – that is with the opening of the hook facing in the direction the horse is facing. Backward hooks can get hooked on the snaffle bit ring and if rider does not notice, by applying the curb rein, he/she will be pulling on the snaffle ring instead of the curb. The hooks’ openings should be just enough wide for the chain to be hooked. Not too narrow and not too wide - just enough. The lower part of the hook should reach the middle of the diameter of the mouthpiece. If the hooks are longer, the chain attached to them will be touching the lower lip and rub, causing a sore on it. As I said, everything should be fit. There are no trifles in dressage. A little pinch will lead to resistance in horse’s behavior. And a rider unaware of that will force the horse to perform without knowing what causes resistance putting the blame on the horse’s stubbornness when in fact it is totally rider’s fault.
Curb Chains
The curb chain should not be too long – only approximately 1.5 times longer than the mouthpiece. Some riders, if the chain is too long, twist the leftovers around the curb. This creates a knot that develops a sore on the horse’s lip, which leads to disobedience caused by mouth pain. Do not jam one end of the chain inside the hook with pliers – if you do, you can only adjust the chain on one side, leaving all of the extra chain dangling (and distracting the horse) on the other side. The curb chain should be untwisted all the way to the right. Make sure it lies flat against the horse’s chin. On most horses, use a chain guard or something soft to protect their jaw. Many horses are very sensitive to curb chain pressure and will resist or fling their heads – in many cases this can be solved by reducing the pain caused by the curb bit. Decrease the pressure and use a soft pad on the chain making it less severe. Using a rubber guard on the chain helps - or simply wrap it in duct tape or something similar.
The chain is adjusted correctly if you could place finger under it and, as we have already discussed, when the shanks of the curb are at an angle of 45 degrees to the line of the horse’s mouth when the reins are pulled. The shorter the chain is adjusted the stronger it is, so for the horses with dull mouth, it should be shorter.