For me, I find that about 1 oz per pound, divided into 2 meals, works for my civvies. Then you tweak for the desired weight - increase to gain, decrease to lose. I'm feeding 15, so I can't be terribly precise!
An 8 lb cat with diabetes, might get 8 oz, plus 4 oz (50% more), divided as half across each of the AM and PM periods.
An 10 lb cat with diabetes, might get 10 oz, plus 5 oz (50% more), divided as half across each of the AM and PM periods.