I have a New Zealand White, a big inactive couch potato:). So I do worry about sludge or stones....so far no issues. "Knock on wood"

I do not limit his calcium but try to offer a balanced diet. I think hydration is VERY important in cases like this. To ensure my boy is well hydrated, I offer plenty of wet leafy greens, at least 5 varieties daily. I always include plenty of romaine lettuce and/or green lettuce for it's high water content, and cilantro which is his favorite. I'll include a higher calcium nutritious green such as baby arugula, or kale etc, alternate dandelion greens ( natural diuretic ) with parsley, and include a lower calcium greens such as escarole, chicory, bok choy, etc.  I try to follow what Dr. Susan Brown suggests in her wonderful article...