I don't really agree or disagree with Roy, but I do want to throw out a disclaimer that can both confirm his statements and call them to quesiton.
My disclaimer is this. We each have different soil conditions and turf conditions. That being said, I have seen on my property that I have totally different conditions in my valley than I have up on the top of my ridgeline. The grass in my valley, if allowed to grow just a bit too long, can easily choke my 60" MMM on a tractor with 18.5 PTO HP. That grass is very thick and lush and while the soil is clay, it is a black clay soil which seems to be fairly rich. The soil on my upper ridge line is a hard tan color which barely supports any plants and what grass that grows up there is sparce and even if it is 10" tall the MMM easily breezes through it without any laboring. I've been to many parts of nation and seen the course gasses of Florida, and the sparce grasses of northern Wisconsin, Michigan and Minnesota and can't imagine ever having those grasses slow down a mower deck. So I generally recommend about 3 to 5 PTO HP for each foot of Finish Mower and 4 to 5 PTO HP for each foot of Rotary Cutter. Bear in mind you have to assess your conditions. If you have thick lush grasses, then go to the upper end of the power requirement scale. If you have space grass, then go to the lower end of the scales. For rotary cutters, if you have a light duty cutter, you could probably go down to something less than 4 PTO HP per food of deck, but only if you are only cutting grasses. I tend to be a bit on the conservative side, but I don't like to bog down my machines any more often than necessary.
JMHO. For a 72" RFM & 60" rotary cutter, I would want about 28 to 30 engine hp.