This may help:

<http://www.purina.com/cats/health/BodyCondition.aspx>

The daily energy requirement for a moderately active, neutered, adult cat is
about 45 kcals/kg or about 20 kcals/lb. Your cat's maintenance DER is about
360 kcals/day.  For a safe weight loss diet, reduce his caloric intake by no
more than 25% or to 270 kcals/day.  If you reduce the caloric intake of x/d
by more 25% of his maintenance DER you'll invade the safety margin of the
diet.  Remember, when you reduce the calories, all other nutrients are also
reduced proportionately.

Another weight loss program you might find easier is simply setting a target
weight and feeding him the amount of calories for that weight-- e.g., his
present weight is 18# and you want him to lose 2#.  His target weight would
be 16#.  Thus, 16 x 20 kcal = 320 kcals/day or about 1.7 cans a day divided
into two feedings.  The advantage of this program is that when he reaches
his target weight, the weight-loss diet will become his maintenance diet
without making any more changes in his caloric intake.  The disadvantage is
that large weight losses must be done in stages so that his caloric intake
is never decreased by more than 25% at a time.

To calculate the amount of food to feed for the first program, divide the
his DER by the caloric density of the diet:  270/1.0275 = 263 grams or 1.4
cans a day.  For the second program, 320/1.0275 = 311 grams/day or 1.7
cans/day.  You need to weigh the food only once to see how many teaspoons =
the appropriate weight.

Now that you have a good scale, you can monitor his weight fairly
accurately.  Allow no more than 1 pound of weight loss every 4 weeks.  If he
loses any more than 1 pound/4 weeks, increase his caloric intake in 10%
increments.

Best of luck,

Phil