What staff-to-resident ration is appropriate for a memory care unit?

I think 1-5 ratio is appropriate, but I doubt if you'll ever see that low of a ratio in practice. My wife was in a well staffed facility for MC patients. More important than the staff ratio, I believe, is the training. Most, if not all, caregiving staff are poorly trained. Most are shown a 4 hour video and then be expected to provide appropriate care. Caring for someone with cognitive issues requires more than 4 hrs. I believe paid caregivers should go thru a minimum of 3 days, preferably a week of training. They are not person centered- competency trained.

Your dad's facility is under staffed. But that's not unusual. I would watch to see how the residents are cared for. When I was looking, at one facility I saw a staff member pulling a wheel chair resident backward to the dining table. I immediately thought "that won't be my wife". So something a simple as that eliminated that facility from my list. So more than the staffing, watch how he is being cared for.

In memory care that RN is likely stuck on phone, record keeping, et al, and taking care of medical crisis. That leaves 2 aids and one med tech. Often med techs are not allowed to do anything BUT meds and these days that is a full time job for 25 residents. That leave two aids making rounds together or separately, as it often takes two to do changes and etc. IF there is not separate staff who comes in for meal time and feeding this 12 patients each or 25 between the two seems very very difficult.
Each facility is different. Is this their norm? Have you spoken with the office? Are they low because of illness? This I am afraid is a serious problem. I think only Califonia, since Schwartenegger, has staffing limits for hospitals. And that doesn't go down to any other care, even skilled nursing. The day the law was passed and I could be required to care for no more than 6 patients was a wonder and a great gift. The one thing a nurse loathes more than any other is not being able to get to their patients who need them.
I don't have any real answers. But you could see this wasn't working, and I don't see, given confusion, danger of falls, and etc, how it could.