Really irritating isn't it when you have to do mental arithmetic in the supermarket aisle to figure out what is in something!
The steps have got to be....
Ask yourself if the ready meal is for one and you will be scoffing the lot in one go.
If the answer is yes then look at the nutrition label and find TOTAL carbohydrate. This might give the total amount in the meal or the amount in 100g of the meal. Sometimes it might give both. If it gives the total amount, then that's what you need. If it gives you the amount per 100g, the find the weight of the meal which should be somewhere on the package, and then do some maths. In your case it was 450g, so you need to multiply the amount per 100g by 4.5. If it was 600g, then you would multiply by 6 and so on.
If the meal is for two and you will only be scoffing half of it then you will need to divide your answer by 2.
Some things to remember.
1. The carb level on the packet is probably the manufacturers best guess and is probably about right. You can certainly ignore the number after the decimal point. In your case you could treat 11.6 g/100 as 10g/100g and not be far wrong. You don't have to work to decimal places.
2. The nutritional info is good for comparing things. If you have got a couple of options, both about the same weight, then simply choose the one with the lowest carbs/100g. That works well with soups. The tins are mostly the same size and you will find big differences in carbs/100g between different brands of the same soup.
3. Supermarket ready meals tend to be up in the high carb range because carbs are a cheap way of getting bulk into the product. Maybe a good occasional standby but generally not the best if you are looking to reduce carbs in your diet.
As others have said, the carb level you aim for and how you get there very much a personal thing. To give you an idea, I aim for about 100g carb/day and so the ready meal you talk about would not be for me because half my target would be used up in one go Getting the rest of my intake in the day under 50g would be an irritation I could do without!