Sounds like you've got it worked out. A plant that big will more than likely be potbound, making it next to impossible to seperate the roots that belong to one division from the others. The only tip I'd give is, instead of seperating the plant into divisions of 5 PBs, if you end up having to cut into the rootball, divide the plant in half this first go-around so you don't end up with some divisions whose cut roots are all stuck to the other pieces, and which are now rootless. I may not have explained that properly, but you'll see what I mean when you go to divide... Also, don't forget to fungicide the cuts. Most of the time they'll scab over without any problem, but I think it's worth taking the preventive measure.