Ihave always taken the instruction about root length to mean , do not pot until they are that long. Otherwise, how do you know ? But I must also add that I ignore it. Pot when new roots are seen, and be very, very sparing with water until growing vigorously, that is the way I do it.
It can be surprising how long a bulbous orchid can go without water, especially a hard bulbed one one like cymbidiums ( and catasetums are in the same generic grouping).