You can use Apps like the following when cooking:
http://apps.nhs.uk/app/cook-and-count-carbs/
If your unsure if you could eat all your meal you can Dual Wave the bolus dose, this means you give some of the insulin upfront and the rest over say an hour or two, so if you don't eat it all you can suspend the remainder of the insulin working on how much food you have eaten.
That's one way, another is to just bolus some of the dose upfront and the rest after eating if you manage to finish your meal.