Hi,
I went to a cross stitch calculator with your information.
Yes you are thinking correctly.
Stitching ‘over 1’ on 25-count fabric will give you the 9 3/4 inch wide x 13 inch high Completed project.
I add 3 inches of extra fabric to each side of of the completed project for mounting, matting and framing, but that’s me, I always use a 3” minimum on each side.
With all the above and figuring in the extra fabric, it came out to you needing 16 inches Wide x 19 inches High fabric.
So the 18 x 27 fabric piece will give you enough fabric.
I’m one of the stitchers on this chat that saw your ‘chat’
Question. :)