How to measure a gullet
As a rule of thumb, you want to measure across the font of the saddle level with the conchos as this is a pretty close indication of where the bars of the saddle are the widest. 6.5" - just under 7" is semi quarter bars, 7" - 8" is full Qtr bars, 8" + is draft.
You can see this last old saddle on the right is a true full quarter into draft size tree with very flat angle to the bars of the saddle. You also want to keep in mind the angle of the bars shown in the last photo under the three saddle photos.
A wide horse will generally require not only a wider gullet but a flatter angle on the bars of the saddle that connect the front of the saddle to the back of the and sit on either side of the horses spine so if the angle is to steep the horse will be uncomfortable, if the angle is to flat on a more steeple shaped horse it will also cause discomfort.
Angle of the bars
Bars of the saddle run front to back and connect the pommel to the cantle and can be flat or more steeple shaped. Every saddle manufacturer is different so it is important to fit your horse with the actual saddle you are interested in buying as not every saddle will fit the same even within the same manufacturer.
I like to do 4 measurements when looking at saddles for a particular horse. Below is the 4 areas I compare, saddle to saddle.
The first is across the arch of the gullet, the second is a bit lower across at the conchos, the third is down lower trying to feel for the bottom of the bars of the saddle and the last photo, you can also feel across the bottom of the saddle and feel for where the bars end side to side and you can see if the bars are wider and flatter at about a 16" measurement, or more medium at 14" or narrower at 13".
Sometimes the saddle skirting will also help by creasing at the bottom of the bars of the saddle so you can see where to measure pretty easily.