
Bagpipe,
To compare protein levels in different foods you need to calculate the dry protein level. You do that by dividing the stated protein level by the food's dry matter content and then multiply by 100. Maths degree is not required, it's a simple formula.

Protein content/(100 - % moisture in food) x 100
Naturediet contains approximately 75% moisture (water) and therefore the its dry matter content is 25%. (Wet foods are 75-80%moisture.)
The protein content in Naturediet works out as follows:
10/25x 100 = 40% protein. (Adult chicken with 10% protein listed on package contents.)
12/25 x 100 = 48% protein. (Puppy variety at 12% protein listed on package contents.)
You do the same for dry foods. I believe Burns is 9% moisture content (dry foods generally have 9-10% moisture). So depending on variety it would work the following for Burns or any food with 9% moisture content:
18.5/91 x 100 = 20% protein (for dry food labeled as containing 18.5% protein and 9% moisture)
20/91 x 100 = 22% protein (for dry food labeled as containing 20% protein and 9% moisture)
21/91 x 100 = 23% protein etc.
24/91 x 100 = 26% protein
That's the formula. Naturediet is a high protein food and much higher in protein to any of the Burns versions and any dry complete foods that I've seen as well.
Trust me, I just went through this with a trainer who recommended I put one of my dogs on a low protein food. Naturediet was definitely not recommended, for the reason above - it's a high protein food.