I just find it easier to use the packaging, or look stuff up on the tesco site (which is where i do my shopping anyway). They list all the nutrients, handily. Every little helps!
Tesco is fine too But if you're unsure of your calorie count then that suggests your sources aren't wholly satisfactory.
But the more important thing is not to stress over it, as there are issues beyond your control, as I detailed above. As long as you're in the right ballpark, then you're ok.