Superior post-surgical feline & canine appliance. (Or: Dump the cone!)

Step 2 Cut the cloth.

Plan to do one or 2 practice tailorings. It's impossible for me to give you precise measurements because each animal is quite different. Even my 2 kitties who are sisters, who are the same age and who weigh exactly the same are different shapes. Buttercup is slim & muscular, while Bella is big & fat.
So, forget your measuring apparatus for now and just "eyeball it". For a medium-sized cat you need a piece of cloth about 20" square. You need 4 holes in it for arms & legs. So, roughly mark this out with your felt pen, as shown in the picture. Now you're ready to cut, but first note the following:
In the picture the cloth is situated so the rump is nearest you and the neck is nearest the recumbent black cat. Be sure to leave 2-3 inches of cloth in front of the arm holes (i.e.: Don't get the arm holes too close to the edge).
Next, the rump. Note that the leg holes need to be CLOSE to the back edge, as close as the fabric can take it without ripping. The reason is that you don't want any cloth sticking out back there in the way when the cat has to pee or poop.
Also note that there is a lot of cloth BETWEEN the leg holes (i.e.:Don't get the leg holes too close to each other). Because here is where the cloth needs to cover and protect the abdominal incision.