I doubt if they are going to have a wicket.
You could get a fairly close ballpark with a tape measure or even a yard stick flat against a wall, then take a ruler and place it on top of the withers. Now the crimp in this is - getting your corgi to stand still for this.
I give 500 mg Vitamin C - pick it up in the supplement aisle of Walmart usually. I'm giving about 1,000 mg G/C to my senior guys daily.
Debbie