Do you weigh your foods? If so HELP! :)

I have been told by my RD to meticulously weigh and measure all food intake and I have been trying to do that. I realize that measuring anything incorrectly can really cause a big problem with your calorie intake. Just letting something go over the top of a measuring cup can add over 50 calories that may not get counted and over a year that will be extra pounds of weight. The RD explained this to me so I really do feel its important to get it as accurate as I can especially since I am feeling very shaky about the exercise calculation part of this. I would love to get one of those arm band things to see for sure what I am burning but can't afford that right now and websites, machines, and apps all give me different numbers of calories burned. I get so confused! I will add 20 minutes of circuit training and it says I have burned 150 calories but I will see posts on this site saying people have burned 3 times that in the same amount of time. I just don't know what is correct.

Now, I am having issues with the food scale. This may sound silly but I had a can of Starkist tuna in water. It was a 5 oz can and it says on the back 2 oz when drained" and serving size is 2 oz with 2 servings per can. So 4 oz per can once you drain the water off right? I drained the water out of this tuna like crazy and it looks dry to me. I split the can in half and put it on the scale expecting it to weigh 2 oz and it says 4 oz! I recalibrate the scale to zero which is a pain because its not the best scale money can buy. I got it for 8 dollars at Walmart. You have to move a dial to get it to zero and it moves off very easily so you have to watch it carefully and fix it each time you weigh something. Also, I weighed a bunch of other items in the kitchen that had its weight listed to check the scale's accuracy and it was spot on. So, I have to assume this tuna measurement is correct. How can half a can be 4 oz on the scale when the can says it should be 2 oz? All I can think is I am not draining off all the water but I can't get any more out of this damn tuna! lol This is all making me crazy. Is there some kind of rule with foods you drain water off of? I am not having this with any other food but tuna and its a big part of my diet. My husband has a digital postal scale I was thinking of using to weigh my food. A scale is a scale right so that should be okay shouldn't it? I don't have access to it yet but I want to get my hands on it to compare it to my manual scale and see if they agree. Especially about this damn tuna thing. I just want to make sure I can at least get the food side of things correct since the exercise is so ambiguous right now. Thanks for listening and if you have any ideas or feedback I would love to hear it.