Hi Phelps,
Let me clarify your questions.
Orlistat, the active ingredient in alli works by inhibiting the fat digesting enzyme lipase. One 60mg dose of alli will block about 25% of the fat consumed. For optimal results you should be eating well balanced, reduced calorie meals where 30% of the calories come from fat (about 15 grams of fat per meal).
Depending on what your current weight is, you will find what your total daily caloric intake should be. Bottom line the to lose weight you need to create a caloric deficit with a combination of consuming less and burning more. You will be looking at both the calories and fat grams.
The difference: 300 calorie meal where 27 calories come from fat (3 grams) about 10% of the total. You will block less than 1 gram of the fat or less than 9 calories in that meal.
300 calorie meal where 90 calories come from fat (10 grams) about 25% of the total. You will block 2.5 grams of fat or 22.5 calories.
Most people choose to skip a dose if the meal contains less than 5 grams of fat just to save the cost of a dose, but alli can be taken even if there is no fat in the meal with no ill effect. It is important though not to be going over the fat limits, since the more fat that is blocked the more that needs to be eliminated (treatment effects).
Hope this helps.
Sophia De Monte RPh