The problem with high protein block is, over time, the kidneys have to work harder and harder, and that will start skin and fur problems.
If you aren't using them for very long, ie-feeders, no biggie. If you notice bald spot or them itching all the time, it could be too much protein..
I used to use urea free cattle cubes years and years ago. Took me a while to figure out why my rats were always having problems, urinating blood, loosing hair, etc. That was back before lab blocks were readily available to the public. When I switched over to hog feed, they would slowly waste away from lack of protein. I would buy stuff from the farm store, and it usually didn't have the info on the labels like today's stuff does. Some cattle feed is as high as 40%, and some hog feed is as low as 12%. You just gotta find a happy medium if you're gonna use that kind of feed. Nothing wrong with it, as long as you can keep your breeders healthy.
I've just "been there, done that", and I take advantage of the feed that's made for lab rats.... Way easier than worrying about mixing and how much of this and how little of that. In the end, at least for me, buying a ton at a time, saves me the time and it's almost as cheap as getting hog feed and supplementing...