the answer to your question, or the range of answers ..

.. depends on how much Jack weighs, and a little history behind that too
is he at his optimum weight, underweight or overweight? and has there been any change in that recently or even in the last year?
first pass through -- all our cats dealing with diabetes are initially hungry all the time, because their cells can't utilize the glucose (fuel) in their blood without insulin to "unlock" the cell walls and let the glucose in -- so cat sputters more or less like old car running on fumes --
HUNGRY!!!!
if he's underweight (as Catcat was, normally a 14 pound cat, he was 10 pounds at diagnosis, terribly skinny), then you can feed him more than a normal cat of his optimum weight -- it's fun ! you still can't allow gluttony though
if he's normal weight, still has a "waist" and has good muscle tone, the rough guideline is 20 calories per pound per day, a little more if he's an outside cat or very active for his age
if he's overweight, I'll need to defer to more experienced members here, you want him to lose the excess SLOWLY -- obesity can be one of the factors that kicks off diabetes, in the same breath you need to keep him well away from ketoacidosis, where two of the risk factors are not enough food, not enough insulin, and often some inflammation or infection going on
despite the puffery in the media, cats do NOT do well on the Keto Diet