Easy way to figure out food: take your stat (after gear bonuses), multiply it by the percentage (which is the number divided by 100, so 5% is 0.05), and if the resulting number is higher than the cap, set it to the cap. That's how much you'll get from food.

The market boards are a good place to do research. The food on there is organized first by category (which stat it raises), then by level (lower level food has a higher % but lower cap, which means you should use low level stuff at low level, and vice-versa). Find the category you want, and find which one gives the most in the stat (Usually the last one you hit the cap on, or the one just after)