When a recipe has both metric and American standard/volume unit of measurement, I prefer to use metric and a scale. I use the information from the back of a products Nutritional Facts to determine weight. For example, unsifted AP Flour states 1/4 cup = 30 grams. Therefore 1 1/2 cups of this flour should be 180 grams.
It is not unusual for the published recipe to list a metric weight that is greatly different from my calculations. This weekend I baked the Momofuku Milk Bar Fruity Pebbles Cookie recipe which calls for 240 grams or 1 1/2 cups of flour. I trust MMB's measurements and assume this is not a misprint. When baking, 60 grams is a substantial difference. Many of you cook using the metric system, so how do others deal with this problem? If I can't trust the manufacturer's information, is there a very reliable and accurate place I can go for specific product information, other than creating my own spreadsheet? I also find happens often with ingredients like olive oil or yogurt. Product info will say 15 grams/ tablespoon 1 cup =240 grams. But 240 grams ends up being more like 1 1/4 cups volume. BTW, I ended up splitting the difference and using 210 grams of flour. The cookies rocked but needed a little more flour - 30 grams would have done the trick.
