Not to derail your initial line of questioning, but why do you need to have your cat on a low-protein diet?
If your cat has been diagnosed as CRF/CKD (kidney disease) and your vet told you that your cat "had" to be on a low-protein diet because it would be "easier" on her kidneys, I can tell you that I have not found a single scientific study proving, beyond a doubt, that a low-protein diet is "good" for cats.
In fact, there are studies that show that it is actually a horrible thing for cats. Cats are obligate carnivores. They NEED protein. If you put a cat, especially an older cat, on a low-protein diet, you will see weight loss, and muscle loss as the cat literally "wastes away". Some of the "low-protein diet" studies showed lower kidney values on the cats fed lower-protein diets, but the cats sometimes suffered from weight loss, muscle loss, anemia, and malnutrition!
My own cat was diagnosed as CRF in 2012, when she was 14. My vet gave me the usual spiel about how she "had" to be on a prescription diet, or she would die in approximately 5 minutes (that was how dramatic the vet made it sound).
I put my cat on a fully-raw diet instead. She is now 17, with kidney values lower than they were when she was diagnosed 3 years ago. She's spunky, happy, has a great coat, and she is enthusiastic about eating. Her phosphorus and potassium levels are perfectly fine.
I'm not trying to demonize those "prescription" low-protein diets, nor am I trying to soapbox too hard about a raw diet, but I do personally believe the prescription foods a load of crap. Vets are paid commissions to sell them to their clients - and of course the only place you can actually BUY prescription diets are from a vet! It's a win/win situation for them. Personally, my vet tried to get me to buy all the Royal Canin and Hill's/Science Diet "renal diet" foods. My cat hated all of them and refused to eat more than a bite or a lick.
Even if your cat is not CRF, Tanya's CRF site offers some good insights into cat protein requirements here:
Tanya's Comprehensive Guide to Feline Chronic Kidney Disease - Nutritional Requirements of CKD Cats
"Dr Zoran believes that healthy cats should eat a diet containing around 45% protein on a dry matter analysis (DMA) basis. ... This level of intake may not be sufficient for older cats. In "Feeding old cats", Dr Sparkes states that older cats need more calories than younger cats, preferably in the form of protein."
The page goes into the low-protein debate very thoroughly and offers insights on both sides. I really think you should re-consider a low-protein diet for your kitty.
Tanya's site also has very comprehensive tables on cat food, listing the protein %s of many common canned and dry foods:
Tanya's Comprehensive Guide to Feline Chronic Kidney Disease- Canned Food Data USA
Tanya's Comprehensive Guide to Feline Chronic Kidney Disease- Dry Food Data USA
The charts give protein as a "dry matter basis" analysis, so you should consult this page to really understand what this means - the %s and info on the cans themselves aren't exactly accurate on a dry-matter basis.
Tanya's Comprehensive Guide to Feline Chronic Kidney Disease - How to Use the Food Data Tables
I know it is SUPER overwhelming and there's a ton of info and it's kind of like a tidal wave all at once. It was the same for me when my cat was diagnosed CRF - I was overwhelmed and terrified my cat was about to die. But I did some research, joined a mailing list group, and slowly came to my above conclusions.
In terms of canned food - personally, when my cat is feeling picky or when I have to be away for a few days (my family refuses to feed her the raw food), I give her a rotating diet of canned Weruva, Ziwipeak, Dave's pet food, and Natural Balance (all non-fish varieties). I believe these brands are very high-quality and are overall very good. Other good brands are Blue Buffalo and pretty much most of the grain-free "limited ingredient" foods out there.
I used to feed Merrick canned as well, but recently it looks like a lot of their recipes have changed (read the reviews on the individual varieties on the Merrick official webpage). It looks like they've opted for cheaper "filler" ingredients now, which is sad.
Honestly, the TL;DR of all of this is: give your cat what she WILL eat. It is far more important for a cat to eat SOMETHING rather than nothing, especially for elderly and/or chronically ill kitties. I sometimes give my cat Fancy Feast classic varieties when she isn't feeling great and won't touch anything else. If your girl won't eat the Royal Canin, give her what she WILL eat. It's better to risk higher kidney values (if she is CRF) than to risk hepatic lipidosis.
Try to transition your girl off of kibble, especially if she is CRF. Wet foods are so much better, plus you can add even more water to the wet food to give your kitty even more fluids.