Feeding my new adoptee

I adopted an obese 8yo male from our local humane this weekend. He weighs in at 18 pounds. He went in for his vet check up today, and aside from telling me that he needed to lose some poundage, the vet was reluctant to tell me how much he should be eating because of different caloric content on different foods. She took his measurements and gave us a chart that showed his target weight is 10.5 pounds.

Kitty is a pretty sedentary indoor-only cat. If I'm reading the right resources, and doing the math right, because of his low energy needs he should probably eat 15-20 calories per pound, per day, which comes to 270-360 calories/day total...? I assume this is to maintain his current weight--what should we aim for to lose weight healthily? I'm assuming a healthy weight loss is a matter of ounces/week (up to 4oz)?

Since we got him, we've been feeding him twice a day, 3oz wet food per meal. We've also left out a half cup of dry food, which he barely touches and really only goes after late at night, eating maybe 1/3 of what's there in a day (I'm starting more precise measurements of this today rather than eyeballing it).

We feed him Simply Nourish, since that's what he was eating before. I can't find kcal ratings on the poultry-based wet that I'd prefer to give him (if anyone knows, please share!) but the seafood bisque flavor is labeled 63 kcals/can for a 3 oz can. Do poultry-based cat foods generally have a higher caloric content? I'd assume they do at least slightly. The dry food is 356 kcals/cup.

So if he eats 2  approx. 63cal cans/day, plus around 60 calories dry/day, plus one 2 cal snack, that's putting him just under 200 calories/day, which from my projections above and general (s)mothering tendencies seems too low. Math tells me that would be ideal for a 13-ish lb cat.

Are we currently feeding him enough to be healthy or healthily lose weight? Should we be feeding more or less? I'm worried about him developing fatty liver disease because he did have it once before and got subsequently placed into in foster care (but made a full recovery due to his loving fosters). He doesn't beg, he doesn't hang around his dish, and he willingly drinks plenty of water.

Thanks in advance for any advice-- this concerned momma appreciates it!

I'm not an expert on cat weight loss, but I do know you want to go slow. Aim for 10% weight loss per month.

You want to give a high protein/low carb diet, so I would ditch the dry food, he doesn't seem to want that much anyway. Replace it with wet. Don't buy weight loss food, that is just usually higher in carbs.

I think 15 cal/pound is the target for weight loss, 20 cal/pound is for maintaining weight.

Just estimating--I know a 10 pound kitty eats on average 4-6 oz. wet food a day, with an average 5.5 can being around 165 calories. So if we take an middle-of-the-road estimate of 5 oz food for 10 pounds, that would be 92% of the can or 151 calories. 18 pounds is 180% of 10 pounds, so 1.8*151=272, pretty close to your estimate.

Like you, I would assume the chicken has somewhat more calories than the fish.