If you're utilizing a steel and HT approach that lowers Mf you probably need cryo. D2 and stainless are probably going to need it - 1084 probably not. O1 - open to debate.
In order for it to work best you want to eliminate things that can cause it to stabilize. For many steels that means it needs to be a part of a continuous quench. So don't delay unnecessarily.
Some steel and some geometry should receive a snap temper to reduce brittleness and chance of cracking. The price will be more RA.
My D2 goes from 1850 to -100 in one smooth drop. But, when treated the way I like it, it has a tendency to retain pretty badly if I don't. The theoretical Mf for D2 is frequently shown down around that value, and my experience has shown little difference going to -300 with D2. I use dry ice because I usually do large batches, not daily use.
I don't understand why time at that temp makes a difference (we're not austenitizing) but that is the general consensus.
In theory you can measure RA with a calibrated magnet, but I wouldn't know where to begin with something like that.
