Cat BMI follow up

I've combed the Internet and spoken to several vets and evidently there is no universally accepted way to calculate cat BMI. The closest is a method involving back leg circumference and back leg length. It's totally stupid because cats store fat around their stomach. It's also surprisingly hard to figure out where a cat's leg starts. The hind appendages merge quite smoothly with the cat butt. The feeding direction on cans of cat food suggest 1 oz of food per pound of cat. My cat weighs 20 lbs and a can of cat food only holds 5.5 oz so my cat would need to eat 3 1/2 cans of food a day. Clearly that's too much cat food for me and I think too much for my cat. Humm...