Hi there. I'm sorry to hear about the diagnosis.
My kitty is also in early stage renal failure (she's 16.5 years old). The BUN/Creatinine ratio is not the same as the BUN (blood urea nitrogen), though I don't know how important the ratio is. So the BUN is high, though it doesn't seem terribly high. But you need to know what the "normal" range is for the lab that did the testing; every lab's reference range for normal is slightly different. If you have the actual lab results, it will show the range.
Did the vet do a urinalysis as well? My vet said that the best way to estimate the stage of CKD is to look at the test results in conjunction with the urinalysis results (looking for the specific gravity).
Are you familiar with Tanya's site? It has an incredible amount of information, and I don't know anyone with a CKD kitty who wouldn't recommend it: Tanya's Comprehensive Guide to Feline Chronic Kidney Disease - Everything You Need to Know to Help Your Cat
The website also has a chart comparing the phosphorus (and other) levels of a ton of different wet foods, both rx and non-rx: Tanya's Comprehensive Guide to Feline Chronic Kidney Disease- Canned Food Data USA
Another site I consult often, to learn about bloodwork results: Normal Dog and Cat Blood And Urine Chemistry Test Results
Did the bloodwork include the SDMA test? It allows for earlier detection than through regular bloodwork, but I believe it's only offered by Idexx, so the bloodwork would need to be sent out to them.
The bad news: "earlier" detection still means a significant percentage loss of kidney function. Regular bloodwork won't necessarily indicate kidney disease until about 75% loss in function, because that's when the creatinine and BUN levels change. The SDMA test is supposedly able to catch it at 40% loss. But as you see, that's still a lot of lost function, and I'm sure you know that it can't be reversed. But the early detection means you can start taking steps earlier, like switching to a low phosphorus diet as you have, and encouraging more water drinking, earlier, and that could prolong life significantly.