I've been refraining from commenting just because I don't feel like the rack pull is an effective tool for this approach. The point of contact is with the bar rather than the plates, meaning that the mechanics tend to be pretty different compared to a deadlift. It also has the issue of being incredibly jarring on the eccentric, and makes it really difficult to have any 2 reps be the same, since the starting position tends to shift.
If you want to use ROM progression as a method, I would find a way to make the plates be the point of contact. I like using rubber patio pavers, but AnytimeJake cut up some boards and stacked them on top of each other. I've known other folks that pulled off of bumper plates. Just something where you can make small gradual changes.
Directly addressing the topic, in my experience, you'll want to be able to pull a fair number of reps from a higher position with a weight if you want to eventually transfer it down to the floor. Usually, the heavier the weight (respective to your 1rm), the more reps you lose transitioning from below the knee to the floor. If I pull a 12rm, I might not lose a single rep, but if I start at my knees and can only pull 7 reps with the weight, I'll be lucky if I can pull it for a double by the end of the cycle.
If your goal is to pull 220kg off the floor, you're probably going to have to be able to pull it about 6-8 times from below the knee.