I'm mainly a western rider but play around English from time to time. I was told that I need to be able to fit 3 fingers in the channel on the underside of the saddle.
The finger fit thing is referring to height & also width.
Same as with a western saddle, you don't want the pommel to sit on the horses wither but to be slightly above it with clearance.
Saddle fit English can be as tricky as western.
You want the saddle channel/gullet to straddle the spinal processes not sit on them.
A bit of extra room either side of the bony spinal column is desirable, and all depends upon how your particular horse is built for a good & comfortable fit for your riding partner.
There really is no "standard" measurement like more commonly found in human apparel.
Trial and error often is the norm.
Try to borrow a few saddles for fitting and to see if you like the ride feel.
Don't limit yourself to a single manufacturer if at all possible, you might miss a comfy perfect match for you and the horse.
When searching for a saddle also remember that all those who have these saddles with change-able fronts...aka "adjustable gullets"... only the pommel changes not the entire channel/gullet of the saddle.
A saddle must
fit the entire length of spine and the saddles underside properly for the horses comfort.
To many forget that changing out the gullet plate does not, repeat does not
alter the rest of the saddle underpinnings.
Many deceive themselves that the saddle is a good fit... and may not be.