I always thought that the R25 absorbed 2 1/3 stops, I guess I was mistaken.
The way I see it, if I'm shooting unfiltered HP5+, and I have a given exposure of 1/125 at ƒ/4, then popping that filter on puts me somewhere around 1/125 at 1.8. I shoot a lot with a 105/1.8 and a 35/1.4, and don't always want to be so close to wide-open, so I push the film back up 2 stops to get me back into ƒ4 or ƒ5.6 territory. Either way, those negs, processed subsequently in either full strength ID-11 or Acufine have quite a unique look to them. Skin tends to go very light, often just a hair short of blowing out, and any clothes people are wearing (I'm shooting in NYC, where people mostly wear blue, black, and grey), get thrown down into the low end. With all the grain, it's a look similar to etchings, which I love.