1 ml of 200 IMCU/ml rennet in 6 liters of milk is 33.33 IMCU per liter, so yes that's a bit low for the style of bloomy right Caldwell is documenting. Incidentally, that recipe is the only one in the book that I think is a bit wonky. There are clearly some mistakes in it but I haven't quite figured out what she was *intending* to do... Anyway 1.5 ml, I think will me just slightly more than you need, but not crazily so. I usually go for 46 IMCU per liter and the standard is 40, but of course different recipes will be slightly more or less than that.
Keep in mind that flocculation time, assuming everything is working as intended, is a factor of rennet amount, pH and temperature. So if you want to decrease the flocculation time a bit, you can increase the ripening time, or you can increase the temperature a bit. Looking at my notes, it seems that I like 34 C, a ripening time of 1 hour (with mother cultures, so they bacteria are already going) and I still get a flocculation time of 15 minutes.
Which brings me to an interesting point. What is the flocculation time? Because generally you float and spin something until it doesn't spin any more. But I've realised that the size of the thing you spin makes a rather large difference. You get more torque with a larger object. I was getting quite variable flocculation times -- out by as much as 10% or so. So I now always use a soda cap (which is small) and I leave it until it leaves a mark in the curd. I think this is a bit longer than most people use for flocculation time, but I've found that it is much more consistent.
Even with 33 IMCU per liter, 25 minutes is longer than I would expect for flocculation time. I suspect it was actually a bit faster. It doesn't have to be much faster to make quite a big difference. Let's say it was actually 24 minutes (which makes math easy :-) ). A 60 minute rest would be a factor of 2.5. A factor of 2.5 will often give you a "clean break". So it's actually completely reasonable to me that you got one at that point.
The idea of the multiplier is that for Caldwell's recipe, you are going well *beyond* a "clean break". However, I think her multiplier is off here... Although I'm not 100% sure. At least in my printing of the book, she actually says different multipliers in different places and neither of them make sense to me. In fact, I've tried a factor of 4 and 5 and both were actually a bit too much IMHO. But it makes a difference how you cut and how you stir and various things, so who knows...
Anyway, back to your bloomy rinds. I'll say that your ~2.5x multiplier will be 100% fine as long as you treated the curds with kid gloves. You may be missing some moisture at the end of the day and if so, your yield will be down and your cheese will be less runny, but it will be fine. Bloomy rinds are actually incredibly forgiving, I've found. Definitely don't go crazy jacking up the rennet. Something like 1.4 ml is probably about right -- possibly a bit less, but I don't think I would go more. If you were really getting a 25 minute flocculation time, I would keep my eye open on things like making sure to dilute with water that is *definitely* free of chlorine (this has caused problems for me!), etc. But also pay attention to the rest of your make -- was your starter culture amount correct, was it in good shape, did you give it enough ripening time, etc, etc.