That the saddle fits the horse is extremely important because a saddle causes many injuries, often permanent, as well as behavioral problems on the horse. For a saddle completely adapted to your horse, you should order a tailor-made saddle completely according to your horse's measurements. You should choose a saddle with a flexible plastic or leather bar or with a bar that is adjustable because horses change shape.
I myself studied and studied saddle fitting and all of its complicated components and learned that there are MANY details that must match the horse's body shape in order for the saddle to truly fit. Below you will find how to measure boom width, boom angle, spinal canal, saddle length (plant surface) and your seat size for the correct saddle size.
Measure boom width & boom angle
The front vault/bar must have the same shape and width as where it will sit on the horse.
For this you need to locate the horse's scapula. Slide your fingers backwards over the bow blade until your fingers dig into the upper edge of the scapula bone. Then move your fingers upwards until you locate the back top corner of the leg.
Then you mark a point three fingers (5 cm) behind this point of the tip of the bow blade. (This is because there must be room for the shoulder blade to rotate upwards/backwards with the horse's movements).
Now take a Flexicurve, (a bendable measuring instrument), or a thick malleable wire or wire hanger to measure the shape and width of the horse. A flexicurve can be bought in equestrian shops or office supply stores. Mark out the center and carefully place it over the horse right at the mark you measured in the position where the front part of the boom is to be placed.
Press the Flexicurve firmly to bring out the shape of the horse and lift it off with the shape maintained. Place it on a cardboard or piece of paper and trace the inside of the shape. Now you have obtained the boom width and boom angle. 
Measure back length/saddle length (Plant area)
Locate the last 18th rib (16th in Arabic) of the horse. Follow it up to the attachment in the spine and mark the position with chalk or masking tape. Measure between these two points you measured front to back, to get the correct saddle length (installation surface). 
Measure spinal canal
The spinal canal of the saddle is the free area between the panels/bosses. The spinal canal must be wide enough not to lie and press on the horse's spine and ligaments. Feel with your fingers where the withers meet the back, from top to bottom at the side. You have to press quite hard and you will feel a bit down that it becomes softer.
Make a mark and the same on the other side and measure between these measurements. You can also take the measurement right across the back to get the width of the horse's spine. Feel with thumb and forefinger and press wider and wider down the side until you feel softness, the muscles.
Mark with a chalk outside the finger on each side. Measure between these two markings to get the back measurement. Standard measurements of saddles can be 5 cm, 7 cm or 9 cm.
Often the channel is made far too narrow or tapers off at the back of the saddle. This causes the saddle to end up on the horse's ligaments in turns and cause discomfort. Most of the time, you should choose 9 cm and it may still be too narrow. Western saddles rarely have this problem.
Seat size
To get the correct seat size for your saddle, measure between your knee joint and hip joint.
Now you have really good measurements to get a saddle that fits your horse as well as it can. If your horse has a curved or hunched back and you also want a measurement of the horse's back, you can also measure the following. 
Back shape
Horses have different shapes on their backs such as curved, hunched or straight backs.
If your horse has a curved or hunched back, lay the Flexicurve horizontally along the spine of the horse and trace this shape so the saddle maker can stop the saddle correctly.
Rear back measurements
At the mark you have reached at the last rib, place the Flexicurve or a malleable piece of steel/cable approx. 60 cm long right over the back and push the shape forward. Lift off and draw off the inside and cut out the triangle. Now you have a measure of the horse's back shape.
A custom-made saddle takes time to make. You can count on around three months. However, it is well worth the wait. You get a saddle for life that really fits your horse and doesn't cause any damage to it.
This saddle is Spanish and I ordered it to measure from Ludomar, a fine Spanish saddle maker. The saddle has a flexible plastic boom.