I think people overcomplicate things sometimes. I go for a color between dull red and dull orange and hold it there for a minute or 2, depending on thickness. Then quench in maize oil. Maize, because that was the cheapest.

This works for O1 and 52100. I realize that this way, I cannot say whether the hardness is going to be 60 or 61, but from a practical pov, who cares? The test I did last weekend proves that this method leads to hardness and toughness that is incredible for 52100, despite the simple heat treatment.