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.