Fiber gets calculated as ZERO calories on fitday. So, if you list 19 as Fiber, it will probably still give you an error. Splenda isn't a problem. Just put it in as 1 Calorie and 0.25g Carbs. No Fiber. If you wish to count it as 1g of Carbs, you'll have to list it as 4 Calories.

calories = (4 *(carbs - fiber)) + (4 * protein) + (9 * fat)

The actual number are slightly different. But, most people just round 'em off to 4-4-9.

You can also use (for slightly better accuracy:

x - y - 4.28361z
----------------- = (carbs - fiber)
3.84063

x -- Total Calories
y -- Fat Calories/Calories From Fat
z -- Protein (grams)