It's tough to figure this out, but I have an idea when XTrain comes for what I want to do. I find HC really works my shoulders a lot, so I'm planning on trying to keep at least one day of HC a week, but separate it from the chest and shoulders disc by at least 2-3 days. I'm planning to look at the XTrain user guide when we get it, since I know Cathe has an XTrain/LIS 90 day rotation in there. My thought proces is this: LIS has lots of horizontal work as well (esp AB, CSS and AT) in the form of plank work, push ups and burpees/mountain climbers with the discs. I figure wherever Cathe has one of those LIS workouts, I can plug in HC potentially. We'll see how it goes!
I do think it's going to be a very individual thing to figure out. Everybody has different tolerances regarding how much they can work a particular area or how intense they can go. One thing I know, is I really don't want to give up HC completely while I do XTrain! The core strength gained from these workouts is unparalleled!