The simplest way is to alter the number of servings. Say for example the value returned is for 4oz and you want to eat 8oz. When you add to the diary say you are having two servings (i.e. double) and you will get the value for double.

(Remember half or double something does NOT equal a half or double value, the formulas used are not linear).

For more complex scenarios or more accuracy use the Advanced Calculation fields of Per & Portion when adding the individual item to the diary or in the calculator

Using the Per and Portion fields you can enter the nutritional information shown on the back of a package to get the value for the amount you actually want to use.

Per = the value given for the nutrionial information.  E.g. Per 4oz , Per 100g

Portion = the serving size you wish to consume.

So if the packet has the value per 16oz and you wish to only eat 4oz you enter 16 in per (As the nutritional given is PER 16oz) and then you enter 4 in the portion (the amount you want to eat) and the app does the rest.

Should you want to change the portion size for an item in the database, find an entry which includes the nutritional information (most do), add it to your favorites, then go to Manage Favorites. 

Choose the item in the list and choose to edit it and enter the corresponding values in the Per & Portion fields and the  app will do the rest for you.

You can then edit the description and hit save and you got everything you need.