http://www.wellbeloved.co.uk/the site will tell you the protein percentages of the various formulae according to recipe and "life stage".
To compare with Burns (or any other brand) you need to know the moisture content of each (as obviously there is no protein content in water) to calculate the "real" protein content.
After that you need to compare the source/quality of the material to understand if they are comparable and equally bio-available to your dog.
HTH