Getting grounds in my percolator coffee, what am I doing wrong?

I grew up with my grandparents using a percolator every morning, so I picked up a COLETTI  Bozeman for nostalgia. Flavor’s great when I get it right, but I occasionally get grounds settling at the bottom. I’m using pre-ground store coffee  wondering if that’s the issue? Do most of you grind specifically for percolators? If so, how coarse are we talking? Would love tips for a cleaner cup without losing that bold percolator flavor.