OK.....dead simple.
Get a carb counter book from somewhere like W H Smiths, about £3.99
A cheap set of Digital Scales, preferably with a Tare facility. Maybe a tenner ?
A basic calculator. Most homes already have them.
Find the total carbs from a packet or can. That will usually be for 100g weight of the product.
Divide the number of carbs by 100. That gives you the number of carbs per gram.
Multiply that figure by the amount of your portion size in grams.
The result will be exactly how many carbs are in the portion you plan to eat. Simples......
E.g. Total carbs on can. 47g
Divide by 100 = 0.47g carbs per gram.
Portion size 40g weight. Multiply that by 0.47 = 18.8g carbs in your portion.......viola !
Hope that helps. You use that calculation whatever food you are using.