Since no one seems to be jumping in....
I asked the same question a while back of some local guys and was told 24"is really pushing it. As we had the room, we decided to go with 28" on the layout to be safe, and sure enough, that's fine for the K36 and 37's (PFM?). Whether or not you could get by with less is not something I can answer. Why not rig a short piece of flex and test it out with one of the big girls? Seems like a worthwhile effort?
Best, Otto